Her political career began in 1986 when her father, a representative in the Georgia House of Representatives, submitted her name as a write-in candidate for the Georgia state house. She received around 40% of the popular vote, although she then lived in Jamaica with her husband, Coy Grandison (and their son, Coy McKinney, born in 1985).

In 1988, McKinney ran for the same seat and won, making the McKinneys the first father and daughter to simultaneously serve in the Georgia state house. In 1991, she spoke aggressively against the Gulf War; many legislators left the chamber in protest of her remarks.

In the 1992 election, McKinney was elected to the U.S. House of Representatives as the member of Congress from the newly created 11th District, an area with an African American 64%-majority population and a district reaching from Atlanta to Savannah. She was the first African American woman to represent Georgia in the House. She was re-elected in 1994.

In 1995, the U.S. Supreme Court ruled in Miller v. Johnson that the 11th District was an unconstitutional gerrymander because the boundaries were drawn based on the racial composition of the constituents. McKinney’s district was subsequently renumbered as the 4th and redrawn to take in almost all of DeKalb County, prompting outrage from McKinney. She asserted that it was a racially discriminatory ruling, given the fact that the Supreme Court had previously ruled that Texas’s 6th District, which is 91% white, was unconstitutional.

The new 4th, however, was no less Democratic than the 11th. McKinney was easily elected from this district in 1996. She was re-elected two more times with no substantive opposition.

During the 2000 presidential campaign, McKinney wrote that “Al Gore’s Negro tolerance level has never been too high. I’ve never known him to have more than one black person around him at any given time.” Gore’s campaign pointed out that its manager, Donna Brazile, was black.

McKinney chastised Gore for failing to support the U’wa people of Colombia trying to oppose petroleum drilling near them. In a press release issued on February 22, 2000, entitled “No More Blood For Oil,” McKinney wrote that “Oil drilling on Uwa land will result in considerable environmental damage and social conflict which will lead to greater militarization of the region as well as an increase in violence.” Addressing herself to Gore, she wrote, “I am contacting you because you have remained silent on this issue despite your strong financial interests and family ties with Occidental.”

McKinney protested the result in court, claiming that thousands of Republicans had voted in the Democratic primary against McKinney in revenge for her anti-Bush administration views and her allegations of voter fraud in Florida in the 2000 presidential election. Like 20 other states, Georgia operates an open primary: voters do not align with a political party when they register to vote and may participate in whichever party’s primary election they choose. Thus, relying on the Supreme Court’s decision in California Democratic Party v. Jones, which had held that California’s blanket primary violated the First Amendment (despite the fact that the Court explicitly differentiated — albeit in dicta — the blanket primary from the open primary in Jones), on McKinney’s behalf, five voters claimed that the open primary system was unconstitutional, operating in violation of the Equal Protection Clause of the Fourteenth Amendment, the associational right protected by the First Amendment, and various statutory rights protected by § 2 of the Voting Rights Act.

Until 2000, McKinney served on the House International Relations Committee, where she was the highest-ranking Democrat on the Human Rights Subcommittee. McKinney worked on legislation to stop conventional weapons transfers to governments that are undemocratic or fail to respect human rights.

On June 14, 2000, a part of Memorial Drive, a major thoroughfare running through her district, was renamed “Cynthia McKinney Parkway,” but the naming has come under scrutiny since her primary defeat in 2006.

On October 17, 2001, McKinney introduced a bill calling for “the suspension of the use, sale, development, production, testing, and export of depleted uranium munitions pending the outcome of certain studies of the health effects of such munitions.” The bill was cosponsored by Reps. Aníbal Acevedo Vilá, Puerto Rico; Tammy Baldwin, D-Wis.; Dennis Kucinich, D-Ohio; Barbara Lee, D-Ca.; and Jim McDermott, D-Wash.

McKinney gained national attention for remarks she made following the September 11 attacks in 2001. She asserted that the United States had “numerous warnings of the events to come” and called for an investigation. She enquired in a radio interview: “What did this administration know and when did it know it?” She said that US President George W. Bush may have been aware and allowed them to happen. She made allegations about the earlier president, George H. W. Bush: “It is known that President Bush’s father, through The Carlyle Group, had–at the time of the attacks–joint business interests with the bin Laden construction company and many defense industry holdings, the stocks of which have soared since September 11.” A spokesman for the Carlyle Group rejected her hypothesis. In a statement in April 2002, McKinney told The Washington Post: “I am not aware of any evidence showing that President Bush or members of his administration have personally profited from the attacks of 9-11. A complete investigation might reveal that to be the case.”

In 2002, McKinney was defeated in the Democratic primary by DeKalb County judge Denise Majette. Majette defeated McKinney with 58% of the vote to McKinney’s 42%.

During 2003 and 2004, McKinney toured the US and much of Europe publicly speaking about her defeat, her opposition to the Iraq War, and the Bush administration.

McKinney submitted to Congress two different versions of the same bill, the “MLK Records Act” (one in 2003, the other in 2005), which, if signed into law, would release all currently sealed files concerning the 1968 assassination of Martin Luther King Jr. These records were sealed in 1978 and are not due to be declassified until the year 2028. The 2005 version of the MLK Records Act, HR 2554 had 67 cosponsors by the time McKinney left office at the end of 2006. A Senate version of the bill (S2499) was introduced by Senator John Kerry and was co-signed by Sen. Hillary Clinton. The bill has also received numerous endorsements from former members of the House Select Committee on Assassinations.

The district court dismissed the case, in the judgement stating the plaintiffs had presented no evidence in support of the 14th Amendment and Voting Rights Act claims, and lacked standing to bring the First Amendment claim. It interpreted the Supreme Court’s Jones ruling to hold that the right to association involved in a dispute over a primary — and thus, standing to sue — belongs to a political party, not an individual voter. On appeal in May 2004, the Eleventh Circuit Court of Appeals upheld this result in Osburn v. Cox, assessing that not only were the plaintiffs’ claims meritless, but the remedy they requested would likely be unconstitutional under the Supreme Court’s decision in Tashjian v. Republican Party of Connecticut. On October 18, 2004, the Supreme Court brought an end to the litigation, denying certiorari without comment.

In 2004, McKinney served on the advisory committee for the group 2004 Racism Watch. On September 9, 2004, she was a commissioner in The Citizens’ Commission on 9-11. On October 26, 2004, she was among 100 Americans and 40 family members of those who were killed on 9/11 who signed the 9/11 Truth Movement statement, calling for new investigations of unexplained aspects of the 9/11 events.

McKinney hosted the first delegation of Afro-Latinos from Central and South America and worked with the World Bank and the U.S. State Department to recognize Afro-Latinos. She stood with Aboriginals against Australian mining companies. She was one of the 31 in the House who objected to the official allotment of the electoral votes from Ohio in the 2004 United States presidential election to incumbent George W. Bush.

McKinney has been featured in a full-length documentary titled American Blackout. On May 1, 2004, during her hiatus from office, McKinney was awarded the so-called fifth annual Backbone Award by an advocacy group, “because she was willing to challenge the Bush administration and called for an investigation into 9-11 when few others dared to air their criticism and questions.”

On July 22, 2005, the first anniversary of the release of the 9/11 Commission Report, McKinney held a briefing on Capitol Hill to address alleged outstanding issues regarding the 2001 attacks on the US. The day-long briefing featured family members of victims, scholars, former intelligence officers and others who critiqued the 9/11 Commission account of 9/11 and its recommendations. The four morning panels addressed flaws, omissions, and a lack of historical and political analysis in the commission’s report. Three afternoon panels critiqued the commission’s recommendations in the areas of foreign and domestic policy and intelligence reform. An Atlanta Journal-Constitution editorial said that the purpose of the event was to discuss whether or not the Bush administration was involved in the 9/11 attacks, expressing surprise that McKinney was once again taking on the issue that was believed to have cost her House seat. The Journal-Constitution declined to publish McKinney’s reply. The 9/11 Commission has sealed all the notes and transcripts of some 2,000 interviews, all the forensic evidence, and both classified and non-classified documents used in compiling its final report until January 2, 2009. McKinney’s interest in 9/11 relates specifically to what she expresses as her opposition to excessive government secrecy, which she has challenged with numerous pieces of legislation.

During the Katrina crisis, evacuees were turned away by Arthur Lawson’s Gretna police when they attempted to cross the Crescent City Connection Bridge between New Orleans and Gretna, Louisiana. McKinney was the only member of Congress to participate in a march across the Crescent City Connection Bridge on November 7, 2005, to protest what had happened on that bridge in the aftermath of Hurricane Katrina.

In response, McKinney introduced a bill on November 2, 2005, that would temporarily deny federal assistance to the City of Gretna Police Department, Harry Lee’s Jefferson Parish Sheriff’s Office, and the Crescent City Connection Police Department, in the state of Louisiana. The bill was referred to the House Judiciary Subcommittee on Crime, Terrorism, and Homeland Security, but was not acted on. However, in August 2006, a grand jury began an investigation of the incident. On October 31, 2007, the Grand Jury ruled not to charge anyone. The Grand Jury accepted Gretna Police Chief Arthur Lawson’s explanation, “Some of the people in the crowd acted aggressively and threatened to throw one of the officers off the bridge, the chief said. The shot was fired over the officer’s shoulder and over the side of the bridge.

The Congressional Black Caucus’ Omnibus Bill (HR 4197) was introduced on November 2, 2005, to provide a comprehensive response to the Gulf Coast residents affected by Hurricane Katrina. The second title of the bill was submitted by McKinney, seeking a Comprehensive Environmental Sampling and Toxicity Assessment Plan, or CESTAP, to minimize harm to Gulf Coast residents from the toxic releases into the environment caused by the hurricane.

At the request of McKinney, the Select Bipartisan Committee to Investigate the Preparation for and Response to Hurricane Katrina, chaired by Thomas M. Davis, held a previously unscheduled hearing titled “Voices Inside the Storm” on December 6, 2005.

On November 18, 2005, McKinney was one of only three House members to vote for H.R. 571, introduced by House Armed Services Committee chairman Duncan Hunter, chairman of the Armed Services Committee, on which McKinney sat. Hunter, a Republican, offered this resolution calling for an immediate withdrawal of U.S. forces in Iraq in place of John Murtha’s H.J.Res. 73, which called for redeployment “at the earliest possible date.” In her prepared statement, McKinney accused the Republicans of “trying to set a trap for the Democrats. A ‘no’ vote for this Resolution will obscure the fact that there is strong support for withdrawal of US forces from Iraq … In voting for this bill, let me be perfectly clear that I am not saying the United States should exit Iraq without a plan. I agree with Mr. Murtha that security and stability in Iraq should be pursued through diplomacy. I simply want to vote ‘yes’ to an orderly withdrawal from Iraq.”

McKinney, in collaboration with Rep. Barbara Lee (CA), produced a “Katrina Legislative Summary,” a chart summarizing House and Senate bills on Hurricane Katrina. On June 13, 2006, McKinney said on the House floor that only a dozen of the 176 Katrina bills identified on the chart had passed into law, leaving 163 bills stalled in committee.

In the midst of a media frenzy, McKinney made an apology on the floor of the House of Representatives on April 6, 2006, neither admitting to nor denying the charge, stating only that: “There should not have been any physical contact in this incident.”

In 2007, McKinney moved from her longtime residence in the Atlanta suburb of Stone Mountain to California. In 2015, she received a Ph.D. from Antioch University with a dissertation on Hugo Chávez.

On August 2, 2007, McKinney participated in a press conference in New Orleans to launch an International Tribunal on Hurricanes Katrina and Rita, which she described as an effort to seek justice for the victims of those hurricanes and their aftermath.

McKinney was a Green Party candidate in the 2008 presidential election.

On July 9, 2008, she named as her running mate journalist and community activist Rosa Clemente and clinched the party’s nomination three days later at the 2008 Green Party National Convention.

On September 10, 2008, McKinney joined a press conference held by third-party and independent candidates, along with Ralph Nader, Chuck Baldwin, and initiator Ron Paul. The participants agreed on four basic principles:

On November 4, 2008, McKinney received 161,797 votes, 0.12% of the total votes cast, placing her behind Obama, McCain, Nader, Barr, and Baldwin.

On December 30, 2008, McKinney was aboard the ship Dignity when it attempted to enter the Gaza Strip, which had its coastal area declared a “closed military zone” by Israel, while on a humanitarian mission by the Free Gaza Movement from Cyprus. Aboard were physicians, medical supplies, and activists, including Caoimhe Butterly. The Israeli Navy confronted the ship at night in international waters. Members of the crew claimed that the ship was rammed, gunfire was directed at the water, and the ship was forced to dock in Lebanon after taking on water. Israeli officials claimed that the collision was accidental and occurred after the ship was informed they would not be allowed to enter Gaza and tried to outmaneuver the patrol boat; they decried McKinney’s actions as being irresponsible and provocative for the sake of propaganda.

On June 30, 2009, McKinney was aboard the Greek-flagged Free Gaza Movement’s ship Spirit of Humanity carrying 21 activists including Irish peace activist Mairead McGuire, medical supplies, a symbolic bag of cement, olive trees and toys, when it was seized by the Israeli Navy 18 mi (29 km) off the Gaza coast. It was unclear whether they were in international waters or in Gazan waters, which is subject to the Israeli blockade of Gaza. Although both the Cypriot and Israeli authorities were officially informed the destination was Gaza before the vessel’s departure, according to the Cypriot government the ship “was given permission by the competent Authorities of the Republic of Cyprus to sail off the port of Larnaca in Cyprus on the basis of its declaration that its intended destination was the port of Port Said in Egypt.”

McKinney was held at the Givon immigration detention center in Ramle, until her release on July 5. McKinney initially refused to sign the deportation papers because they were written in Hebrew and that the papers would require them to admit that they were in violation of Israel’s blockade, which they denied. According to The Atlanta Journal-Constitution, Israeli officials stated that the “Palestinian Authority and the rest of the international community had agreed to the off-shore blockade to prevent arms smuggling into Gaza.” The Palestinian Chronicle reports that such an agreement to the off-shore blockade never happened. “No Palestinians have agreed nor did the international community agree to a blockade of Gaza by land or Sea.” On June 17, 2009, a group of United Nations agencies and non-governmental organizations (NGOs) called for an end to Israel’s blockade of the Gaza Strip.

On July 7, 2009, McKinney was deported to the United States. The Israeli government indicated it would deliver the supplies via land.

On May 21, 2011, McKinney appeared on state-run television in Libya and stated that United States participation in military intervention in the 2011 Libyan civil war was “not what the people of the United States stand for and it’s not what African-Americans stand for”. Also on Memri-TV, McKinney stated: “On a previous visit to Libya, I was able to learn about The Green Book, and the form of direct democracy that is advocated in The Green Book.”

McKinney announced in April 2012 that she would run for the 4th congressional district against Hank Johnson on the Green Party ticket. However, in August she failed to qualify for the ballot. Nevertheless, she received 58 write-in votes in the general election.