Gardner was born on August 22, 1974, in Yuma, Colorado, the son of Cindy L. (née Pagel) and John W. Gardner. He is of Irish, German, Austrian, and English descent. He graduated summa cum laude with a Bachelor of Arts in political science in 1997 from Colorado State University, where he was a member of the FarmHouse fraternity.

In college, Gardner switched from the Democratic Party to the Republican Party and interned at the Colorado State Capitol. He went to law school at the University of Colorado to earn his Juris Doctor in 2001. Gardner served as General Counsel and Legislative Director for former U.S. Senator Wayne Allard of Colorado from 2002 to 2005.

Gardner was appointed to the Colorado House of Representatives in 2005 and elected to a full term in 2006. He represented District 63 in the Colorado House of Representatives from 2005 through 2011.

In 2006, Gardner proposed legislation to create a rainy-day fund to help protect the state from future economic downturns. His proposal relied on money made available by Referendum C—which allowed state revenue caps to be exceeded for five years—for future budget emergencies. He staunchly opposed any tax increases. He helped create the Colorado Clean Energy Development Authority which issued bonds to finance projects that involve the production, transportation and storage of clean energy until it was repealed in 2012.

In 2006, Gardner opposed legislation to allow pharmacists to prescribe emergency contraception and proposed an amendment to the Colorado state budget to prohibit the state Medicaid plan from purchasing Plan B emergency contraception. In 2007, he voted against a bill requiring hospitals to inform survivors of a sexual assault of the availability of emergency contraception.

In 2007, then a state representative, Gardner voted against legislation to allow Colorado gay and lesbian couples to adopt children.

Gardner was endorsed by former U.S. Congressman Tom Tancredo. On November 2, 2010, Gardner defeated Markey, 52%–41%.

In 2011, Gardner voted for the Respect for Rights of Conscience Act, which states that “nothing in the Affordable Care Act shall be construed to authorize a health plan to require a provider to provide, participate in, or refer for a specific item or service contrary to the provider’s religious beliefs or moral convictions.”

In 2012, Gardner co-sponsored the controversial so-called “personhood” legislation titled the Life Begins at Conception Act. Gardner later said that he changed his mind on personhood after listening to voters. According to The Denver Post, “Gardner conceded that with his new position on personhood, he might be accused of flip-flopping simply to make himself more palatable to statewide voters.” The nonpartisan Factcheck.org said “It would be clearer to say that Gardner supports efforts to ban abortion that could also ban some forms of birth control. As for his change of position, voters in Colorado should know Gardner still supports a federal bill that would prompt the same concerns over birth control as the state measure he says he rejects on the same grounds.”

In 2012, Gardner voted to renew the Violence Against Women Act of 1994 (VAWA), which reauthorized the bill and expanded protections for Native Americans, immigrants, and gays and lesbians.

In 2012, Gardner opposed Colorado Amendment 64, which legalized recreational marijuana. Since that time, he has come to support legalizing marijuana. In 2019, Playboy called him “the potential cannabis power broker in this Congress.”

In 2013, he introduced a bill to allow the United States Environmental Protection Agency (EPA) to review solid waste regulations at its discretion, rather than automatically every three years. It would also grant precedence to state requirements for solid waste disposal when creating new federal requirements.

In 2013, Gardner announced that he would introduce a bill to prohibit executives of state healthcare exchanges from getting bonuses.

In 2014, Gardner called for over-the-counter access to oral contraceptives and said the birth control pill would be safer and cheaper if it was available over the counter.

In July 2014, Gardner introduced legislation to reform the Earned Income Tax Credit program. The legislation seeks to reduce fraud in the program and dedicate the savings to increasing the credit for working families.

In August 2014, Gardner voted against a bill that would have dismantled the Deferred Action for Childhood Arrivals. He has said that he supports immigration reform in the form of a guest worker program and increased border security.

In 2015, Gardner voted against giving same-sex partners access to the Social Security and veterans benefits earned by their spouses.

In the 2016 presidential election, Gardner initially endorsed Donald Trump. In October, however, after the release of the Access Hollywood tape, Gardner said he would not vote for him, whom he called “a candidate whose flaws are beyond mere moral shortcomings and who shows a disgust for American character and a disdain for dignity unbecoming of the Presidency. I cannot and will not support someone who brags about degrading and assaulting women.” Nevertheless, in the 2020 presidential election, Gardner endorsed Trump.

In September 2016, Gardner was one of 34 senators to sign a letter to United States Secretary of State John Kerry advocating that the United States clearly enforce “a legally binding Security Council Resolution” by using “all available tools to dissuade Russia from continuing its airstrikes in Syria that are clearly not in our interest”.

In 2016, Gardner voted against the Feinstein Amendment, which sought to ban gun sales to anyone who had been placed on the terrorist watch list for the last five years. He also opposed an amendment making it necessary for background checks to take place for guns bought at gun shows and online.

In 2017, Gardner criticized Trump’s response to the Unite the Right rally, writing, “Mr. President—we must call evil by its name. These were white supremacists and this was domestic terrorism.” He has also differed with Trump on some trade and foreign policy issues.

In September 2017, Gardner co-sponsored the Israel Anti-Boycott Act (S.270), which made it a federal crime, punishable by a maximum sentence of 20 years imprisonment, for Americans to encourage or participate in boycotts against Israel and Israeli settlements in the occupied Palestinian territories if protesting actions by the Israeli government.

Gardner was part of the group of 13 Republican senators drafting the Senate version of the American Health Care Act, which would have repealed and replaced the Affordable Care Act. He voted in favor of all variations of AHCA that came up for a vote in the Senate. The New York Times reported that in September 2017, when the GOP made another attempt to pass legislation to repeal the Affordable Care Act, Gardner warned Republican legislators at a closed luncheon that failure to pass any repeal legislation would lead to a backlash by big donors to Republicans, as well as the grassroots.

In March 2017, Gardner voted for the Broadband Consumer Privacy Proposal, which repealed the FCC’s internet privacy rules and allowed internet service providers to sell customers’ browsing history without their permission.

In April 2018, the Lugar Center and Georgetown’s McCourt School of Public Policy ranked Gardner the 8th most bipartisan senator in the first session of the 115th United States Congress. GovTrack noted that of the 157 bills Gardner cosponsored in 2017, 41% were introduced by legislators who were not Republican. Gardner was ranked the 5th most bipartisan senator in 2019.

In January 2018, he signed a letter urging the president to preserve and modernize the North American Free Trade Agreement. In March 2018, he criticized Trump for imposing a 10% tariff on aluminum imports and a 25% tariff on steel imports, arguing that they would lead to a trade war that would threaten the American economy, particularly agriculture. “I am concerned that a tariff can result in a tax on the very same people that we are trying to help in this economy,” he said. In June 2019 Gardner again expressed concern over Trump’s threats to impose tariffs on goods entering the United States from overseas. He argued that such tariffs would result in “a 1.1% tax increase for the lowest 20% of income earners; a 0.3% increase for those in the middle; and a zero net change for the upper middle class.” Gardner said that by implementing harsh tariff policies America would be “turning [its] backs on American workers and consumers.”

In 2018, Gardner and Senator Michael Bennet introduced bills requiring the U.S. Energy Department to identify vulnerabilities to cyberattacks in the nation’s electrical power grid and assigning $90 million to be distributed to states to develop energy security plans.

In April 2018, Gardner was one of eight Republican senators to sign a letter to United States Secretary of the Treasury Steve Mnuchin and acting Secretary of State John Sullivan expressing “deep concern” over a United Nations report that exposed “North Korean sanctions evasion involving Russia and China”, saying that the findings “demonstrate an elaborate and alarming military-venture between rogue, tyrannical states to avoid United States and international sanctions and inflict terror and death upon thousands of innocent people”, and calling it “imperative that the United States provides a swift and appropriate response to the continued use of chemical weapons used by President Assad and his forces, and works to address the shortcomings in sanctions enforcement.”

In September 2018, Gardner was one of five senators to sign a letter to United States Secretary of State Mike Pompeo urging him to employ more multifactor authentication measures to secure the State Department’s information systems and seeking answers on how the department would boost its security following the Office of Management and Budget’s designation of the department’s cyber-readiness as “high risk”, what the department would do to address the lack of multifactor authentication required by law, and statistics on the department’s cyber incidents over the last three years.

In December 2018, Gardner voted against ending U.S. military support to the Saudi Arabian-led coalition in the Yemen war. He said that Saudi Arabia “is a country in a critical part of the region that has played a key role in our work protecting Israel.” In March 2019, Gardner voted against the resolution again, saying it would have empowered Iran. In January 2019, Gardner joined Rubio, Jim Risch, and Senate Majority Leader Mitch McConnell in introducing legislation to impose sanctions on the government of President of Syria Bashar al-Assad and bolster American cooperation with Israel and Jordan.

In June 2018, Gardner was one of 13 Republican senators to sign a letter to Attorney General Jeff Sessions requesting a moratorium on the Trump administration family separation policy while Congress drafted legislation.

Gardner opposes net neutrality, calling the regulations “brazen abuse of power and overreach.” On May 16, 2018, he voted against The Congressional Review Act, a bill to reinstate net neutrality.

In January 2018, Gardner criticized Attorney General Jeff Sessions for announcing a crackdown on marijuana dispensaries around the country, which he said was contrary to what Sessions had told him during his confirmation hearing. In response, Gardner placed a temporary hold on some of Trump’s judicial nominees.

In January 2019, Gardner was one of 11 Republican senators to vote to advance legislation intended to prevent Trump from lifting sanctions against three Russian companies.

In January 2019, following a report that Trump had expressed interest in withdrawing from NATO several times during the previous year, Gardner was one of eight senators to reintroduce legislation to prevent Trump from withdrawing the United States from NATO by imposing a requirement of a two-thirds approval from the Senate for a president to suspend, terminate or withdraw American involvement with it.

In April 2019, Gardner 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 Trump’s 2020 budget request to combine the two federal programs that do carbon capture research.

In February 2019, Gardner was one of 20 senators to sponsor the Employer Participation in Repayment Act, enabling employers to contribute up to $5,250 to the student loans of their employees.

In January 2019, Gardner was one of six senators to cosponsor the Health Insurance Tax Relief Act, delaying the Health Insurance Tax for two years.

In October 2019, Gardner was one of 27 senators to sign a letter to Senate Majority Leader Mitch McConnell and Senate Minority Leader Chuck Schumer advocating the passage of the Community Health Investment, Modernization, and Excellence (CHIME) Act, which was set to expire the following month. The senators warned that if the funding for the Community Health Center Fund (CHCF) was allowed to expire, it “would cause an estimated 2,400 site closures, 47,000 lost jobs, and threaten the health care of approximately 9 million Americans.”

In 2019, Gardner was described by Politico as “reliably conservative on most issues other than immigration.” Gardner is typically moderate on immigration policy; in 2019 the National Immigration Forum, an immigrant advocacy group, honored him and Democratic U.S. Senator Dick Durbin for their bipartisan immigration work.

In March 2019, Gardner voted for Trump’s national emergency declaration on the creation of a southern border wall (which allows Trump to take funding from other government functions in order to spend them on a border wall). Majorities in both the House of Representatives and the Senate (where 12 Republican Senators joined with Democrats) voted to overturn Trump’s national emergency declaration. The Denver Post rescinded its 2014 endorsement of Gardner, citing his vote on Trump’s national emergency declaration.

In 2019, Gardner introduced the National Suicide Hotline Designation Act, which creates a new nationwide three-digit phone number (988) to connect to a suicide prevention hotline. In addition to funding the new number, the bill requires a strategy report for suicide prevention services for LGBTQ youth, minorities, and rural individuals, among other high-risk groups. The bill unanimously passed the U.S. Senate and was signed into law by Trump in October 2020.

Gardner ran for reelection in 2020. According to The Colorado Sun, Gardner “decisively tied his reelection bid to President Donald Trump. Gardner lost his re-election bid to former Democratic Governor John Hickenlooper.

In 2020, Gardner introduced the bipartisan Great American Outdoors Act, which passed with 58 co-sponsors, providing full and permanent funding of $900 million a year for the Land and Water Conservation Fund to aid national parks and public lands.

In May 2020, Gardner voted for an amendment co-sponsored by Senators Steve Daines and Ron Wyden that would have required federal intelligence and law enforcement agencies to obtain federal court warrants when collecting web search engine data from American citizens, nationals, or residents under the Foreign Intelligence Surveillance Act (FISA).

After the death of Supreme Court Justice Ruth Bader Ginsburg in September 2020, Gardner said he would vote to confirm a “qualified” nominee to replace her. He did not say whether he supported holding a confirmation vote before the November presidential election. In 2016, after the death of Justice Antonin Scalia, Gardner said the Senate should delay confirming justices so close to a presidential election.

In 2020, Politico described Gardner as “a champion of the cannabis industry” and “the GOP’s most ardent promoter of cannabis in Congress.” He has sponsored a bill that would increase the marijuana industry’s access to banking and capital and another that would codify federal protections for states that have legalized marijuana.