In 1987, Bennet earned his Bachelor of Arts in history from Wesleyan University, the alma mater of his father and grandfather. At Wesleyan he was a member of Beta Theta Pi. Bennet earned his J.D. degree from Yale Law School, where he was the Editor-in-Chief of the Yale Law Journal.

From 1988 until 1990, when he left to attend Yale, he served as an aide to Ohio Governor Richard Celeste. After law school he served as a law clerk for the 4th Circuit Court of Appeals and as an associate to Washington, D.C. attorney Lloyd Cutler. He then served as Counsel to the Deputy Attorney General during the Bill Clinton administration. Douglas Bennet worked in the Clinton White House as well, as Assistant Secretary of State for International Organization Affairs. Following a stint as an assistant to the U.S. attorney in Connecticut, Bennet left the legal world and moved West. After briefly living in Montana, he moved with his fiancé to Colorado in 1997. Bennet worked for six years in Denver as Managing Director for the Anschutz Investment Company, where he led the reorganization of an oil company and helped consolidate three movie theater chains into the Regal Entertainment Group.

On October 26, 1997, Bennet married Earthjustice Legal Defense Fund attorney Susan Diane Daggett, in Marianna, Arkansas. They have three daughters and reside in Denver’s Congress Park neighborhood.

The Denver Board of Education selected Bennet as superintendent of Denver Public Schools on June 27, 2005, and he took office on July 1. He had no experience as a school administrator. Under Bennet’s leadership, the Denver Public School system grew student enrollment, decreased dropout rates, and improved graduation rates and college enrollment. Those trends have continued since Bennet left the office. Bennet collaborated with educators and community members to develop the Denver Plan, a commitment to increase student success by focusing on higher expectations, better professional learning opportunities for educators, and deeper engagement with the community and stakeholders. Bennet and the City of Denver also partnered with private philanthropists to increase college enrollment and affordability for DPS graduates. The Denver Post said of his tenure, “Bennet has been a force—pushing reforms and steering the state’s second-largest district to a culture of success.”

In 2008 Bennet persuaded the Denver Board of Education to enter into a 30-year, $750 million financial bond transaction with variable interest rates designed to fluctuate as economic conditions changed. According to The New York Times, “In short order, the transaction went awry because of stress in the credit markets, problems with the bond insurer and plummeting interest rates.” As of 2010 the school system had paid $115 million in interest and other fees, at least $25 million more than it originally anticipated.

On January 3, 2009, he was named by Colorado Governor Bill Ritter to fill the seat in the United States Senate vacated by United States Secretary of the Interior Ken Salazar on January 20. Ritter chose Bennet after interviewing several prominent Colorado Democrats, and Bennet took the job with the blessing of Hickenlooper. Upon taking office on January 21, 2009, he stated that he would seek election at the end of his term in 2010.

In September 2009, Bennet cosponsored the DREAM Act (S. 729), which proposed amending the Illegal Immigration Reform and Immigrant Responsibility Act of 1996 by giving residency to immigrants enrolled in higher education programs or serving in the military. In 2013, he was a member of the Gang of Eight, a bipartisan group of four Democratic and four Republican U.S. Senators who introduced comprehensive immigration reform legislation. Their bill, the Border Security, Economic Opportunity, and Immigration Modernization Act of 2013, passed the U.S. Senate with a vote of 68-32, but stalled in the House due to opposition from the Republican majority. He later cosponsored the Dream Act of 2017. After President Trump ended the Deferred Action for Childhood Arrivals (DACA) program, Bennet worked with a bipartisan group of Senators to provide a pathway to citizenship for Dreamers with stronger border protections.

In 2009, Bennet co-sponsored the Solar Manufacturing Jobs Creation Act, legislation that would have provided a tax credit to support solar manufacturing in the U.S. The legislation was not enacted.

Bennet ran for election for a full term as Senator from Colorado in the 2010 election. On September 16, 2009, former Colorado House Speaker Andrew Romanoff announced his campaign to challenge Bennet for the Democratic nomination. Bennet received endorsements from President Obama, U.S. Senator Mark Udall, and U.S. Representatives Betsy Markey, Jared Polis, and John Salazar of the Colorado congressional delegation. He raised $7 million and had a four-to-one cash advantage over Romanoff.

On August 10, 2010, Bennet defeated Romanoff in the primary and won his party’s nomination, facing Republican candidate Ken Buck. The campaign became one of the most expensive in the country, with the candidates spending a reported $15 million combined, and outside groups another $30 million. Bennet portrayed Buck as an extremist conservative opposed to abortion and direct election of Senators, while Buck and the groups supporting him characterized Bennet as a big-spending liberal.

As of 2010, Bennet had earned a “C+” grade from the National Rifle Association for a mixed record regarding his votes for gun rights. In 2012, Bennet joined then Colorado Senator Mark Udall in asking for stricter gun control, in response to the Sandy Hook Elementary School shooting. After the shooting, Bennet said, “In Colorado, we support the 2nd Amendment right to bear arms, we support the ability of people to hunt and recreate and to protect their families and homes, and we want to keep the wrong weapons out of the hands of the wrong people.”

Bennet voted in support of the Patient Protection and Affordable Care Act signed by President Barack Obama on March 23, 2010. In November 2009, when the bill was still working its way through Congress, Bennet said that he would support health care reform even if it meant losing the election. In 2016, describing the healthcare costs in western and central Colorado as among the highest in the United States, Bennet said he “didn’t have answers” and called it “next to impossible” to fix the Affordable Care Act given partisan attitudes at that time.

On November 3, the day after polls closed, Bennet was declared the winner and Buck conceded. Bennet won by 851,590 votes (48.1%) to 822,731 (46.4%). He subsequently returned to Washington, D.C. in January 2011 to start a full six-year term. After the election, Obama said Bennet “perfectly reflects the qualities of the ruggedly independent state he has been chosen to serve.”

In 2013, Bennet voted against a Senate Amendment 711 to S. 649 (The Safe Communities, Safe Schools Act of 2013), an amendment introduced by Senator Dianne Feinstein (D-CA) that would have reinstated the federal assault weapons ban. The amendment was defeated 40-60 with one Republican, Mark Kirk, voting in favor and 16 Democrats (including independent Senator Angus King, who caucuses with Democrats) against. Also in 2013 Bennet voted to strengthen the background check system and to ban high-capacity magazines.

He was one of the handful of Democratic Senators who have supported construction of the Keystone XL pipeline, voting for it in 2013, 2014, and 2015.

Bennet was reelected to a second term on November 8, 2016, defeating the Republican nominee, El Paso County Commissioner Darryl Glenn. Bennet received 1.36 million votes, 156,248 more than Glenn. He received 31,780 more votes in Colorado than Democratic presidential candidate Hillary Clinton, who won the state in the presidential election. Bennet received more votes than any other Democrat in a statewide race in Colorado history. He also won more votes in Colorado’s rural counties than any other statewide Democrat in state history.

In October 2017, Bennet was one of 19 senators to sign a letter to Administrator of the Environmental Protection Agency Scott Pruitt questioning Pruitt’s decision to repeal the Clean Power Plan, asserting that Pruitt used “mathematical sleights of hand to overstate the costs of industry compliance with the 2015 Rule and understate the benefits that will be lost if the 2017 repeal is finalized”, and that science denial and math tricks fail to “satisfy the requirements of the law, nor will it slow the increase in frequency and intensity of extreme weather events, the inexorable rise in sea levels, or the other dire effects of global warming that our planet is already experiencing.”

In July 2017, Bennet co-sponsored the Israel Anti-Boycott Act (s. 720), which permits U.S. states to enact laws that require contractors to sign a pledge saying they will not boycott goods from Israel, or their contracts will be terminated.

In August 2018, Bennet was one of 17 senators to sign a letter spearheaded by Kamala Harris to United States Secretary of Homeland Security Kirstjen Nielsen demanding that the Trump administration take immediate action in attempting to reunite 539 migrant children with their families, citing each passing day of inaction as intensifying “trauma that this administration has needlessly caused for children and their families seeking humanitarian protection.”

In November 2018, Bennet was one of 25 Democratic senators to cosponsor a resolution in response to findings of the Intergovernmental Panel On Climate Change report and National Climate Assessment. The resolution affirmed the senators’ acceptance of the findings and their support for bold action to address climate change.

In March 2018, Bennet voted against tabling a resolution spearheaded by Bernie Sanders, Chris Murphy, and Mike Lee that would have required President Trump to withdraw American troops either in or influencing Yemen within the next 30 days unless they were combating Al-Qaeda.

In November 2018, Bennet joined Senators Chris Coons, Elizabeth Warren, and a bipartisan group of lawmakers in sending the Trump administration a letter raising concerns about the People’s Republic of China’s undue influence on media outlets and academic institutions in the United States. They wrote: “In American news outlets, Beijing has used financial ties to suppress negative information about the CCP. In the past four years, multiple media outlets with direct or indirect financial ties to China allegedly decided not to publish stories on wealth and corruption in the CCP. In one case, an editor resigned due to mounting self-censorship in the outlet’s China coverage. Beijing has also sought to use relationships with American academic institutions and student groups to shape public discourse.”

Bennet has cosponsored the bipartisan STATES Act, proposed in the 115th U.S. Congress by Massachusetts Senator Elizabeth Warren and fellow Colorado Senator Cory Gardner, that would exempt individuals or corporations in compliance with state cannabis laws from federal enforcement of the Controlled Substances Act. In 2018 Bennet criticized the Trump administration for attempting to cherry-pick data to misinform the public on marijuana use. In response, the White House’s Office of National Drug Control Policy committed to be completely objective and dispassionate in its analysis of marijuana.

In March 2019, Bennet was one of 38 senators to sign a letter to United States Secretary of Agriculture Sonny Perdue warning that dairy farmers “have continued to face market instability and are struggling to survive the fourth year of sustained low prices” and urging his department to “strongly encourage these farmers to consider the Dairy Margin Coverage program.”

As part of a group of Democrats proposing “more incremental steps to broaden health care coverage”, as opposed to Bernie Sanders’s push for “Medicare for All”, Bennet and Senator Tim Kaine have proposed “Medicare X”. Medicare X would “create a public option modeled after Medicare alongside private options on the ObamaCare marketplaces”. In 2019 Bennet and Kaine reintroduced the latest version of the plan, which would also “expand access to tax credits.”

In January 2019 during the 2018–19 United States federal government shutdown, Bennet was one of 34 senators to sign a letter to Commissioner of Food and Drugs Scott Gottlieb recognizing the efforts of the FDA to address the shutdown’s effect on public health and employees while remaining alarmed “that the continued shutdown will result in increasingly harmful effects on the agency’s employees and the safety and security of the nation’s food and medical products.”

In April 2019 Bennet and Senator Chuck Grassley’s Advancing Care for Exceptional (ACE) Kids Act, legislation that creates an option for states and families to provide improved coordination of care for children with complex medical conditions, was signed into law.

In June 2019, Bennet and six other Democratic senators led by Hawaii Senator Brian Schatz sent letters to the Government Accountability Office and the suspension and debarment official and inspector general at the US Department of Health and Human Services citing recent reports that showed “significant evidence that some federal contractors and grantees have not provided adequate accommodations for children in line with legal and contractual requirements” and urged the officials to determine whether federal contractors and grantees are in violation of contractual obligations or federal regulations and should thus face financial consequences.

In July 2019, following reports that the Trump administration intended to cease protecting spouses, parents and children of active-duty service members from deportation, Bennet was one of 22 senators led by Tammy Duckworth to sign a letter arguing that the protection gave service members the ability “to fight for the United States overseas and not worry that their spouse, children, or parents will be deported while they are away” and that its termination would both cause service members personal hardship and negatively affect their combat performance.

Also in July 2019, Markey and 15 other Senate Democrats introduced the Protecting Sensitive Locations Act, which would require, except in special circumstances, that ICE agents get approval from a supervisor before engaging in enforcement actions at sensitive locations, and that agents receive annual training in addition to reporting annually on enforcement actions in those locations.

In December 2019, Bennet and Senator Mitt Romney proposed a universal basic income bill.

In March 2019, Bennet was an original cosponsor of a bipartisan bill intended to mandate that the Environmental Protection Agency declare per- and polyfluoroalkyl substances as hazardous substances that can be addressed with cleanup funds via the EPA Superfund law and require that polluters undertake or pay for remediation within a year of the bill’s enaction.

In April 2019, Bennet was one of 12 senators to sign a bipartisan letter to top senators on the Appropriations Subcommittee on Energy and Water Development advocating that the Energy Department be granted maximum funding for carbon capture, utilization and storage (CCUS), arguing that American job growth could be stimulated by investment in capturing carbon emissions and expressing disagreement with President Trump’s 2020 budget request to combine the two federal programs that do carbon capture research.

In September 2019, Bennet was one of eight senators to sign a bipartisan letter to congressional leadership requesting full and lasting funding of the Land and Water Conservation Act in order to aid national parks and public lands, benefit the $887 billion American outdoor recreation economy, and “ensure much-needed investment in our public lands and continuity for the state, tribal, and non-federal partners who depend on them.”

In April 2019, Bennet was one of 34 senators to sign a letter to President Trump regarding cutting aid to Central America. It encouraged Trump “to listen to members of your own Administration and reverse a decision that will damage our national security and aggravate conditions inside Central America,” asserting that Trump had “consistently expressed a flawed understanding of U.S. foreign assistance” since becoming president and that he was “personally undermining efforts to promote U.S. national security and economic prosperity” by preventing the use of Fiscal Year 2018 national security funding. The senators argued that foreign assistance to Central American countries created less migration to the U.S. by helping to improve conditions in those countries.

On January 24, 2019, Bennet gave an impromptu 25-minute speech on the Senate floor in response to comments by Senator Ted Cruz. He questioned the authenticity of Cruz’s concern about difficulties that the 2018–19 government shutdown was causing to first responders, recalling that in 2013 Cruz led a shutdown that lasted 16 days at a time when Colorado was experiencing flooding. In less than eight hours the speech became the most-watched Senate floor speech in C-SPAN history.

On April 3, 2019, Bennet announced he had been diagnosed with prostate cancer and underwent surgery later that month. Following the surgery, Bennet’s office said the procedure was “completely successful” and that he requires no further treatment.

Bennet’s 2020 presidential campaign began on May 2, 2019, when he officially declared his candidacy during an appearance on CBS This Morning. Bennet was previously mentioned as a possible presidential candidate following his viral response to Senator Ted Cruz in January 2019. In February and March 2019 he traveled to early primary states such as Iowa and New Hampshire. In late March Bennet said he was “very inclined” to run for the presidency.

Bennet received 164 votes in the Iowa caucuses, and 958 in the New Hampshire primary. He dropped out of the race on February 11, 2020, the night of the New Hampshire primary.