McKinney was the daughter of Billy McKinney, one of Atlanta's first African American police officers and a longtime member of the Georgia House of Representatives; the elder McKinney was known for his fiery spirit and his sometimes controversial views. After earning a B.A in international relations from the University of Southern California (1978) and an M.A. in law and diplomacy from Tufts University (1979), Cynthia followed him into the Georgia statehouse in 1988. She served two terms as a state representative, and in 1992 she ran successfully for the U.S. House of Representatives.

During her tenure in Congress, McKinney served on the armed services and international relations committees. In 1995 the U.S. Supreme Court (Supreme Court of the United States) ruled that McKinney's legislative district violated constitutional prohibitions against gerrymandering. The district was redrawn and she handily won reelection, and she directed her legislative efforts toward such subjects as poverty, racial issues, and the relief of debt in the developing world. In April 2002 she courted controversy when she openly speculated that the administration of Pres. George W. Bush (Bush, George W.) might have had foreknowledge of the September 11 attacks. Backlash against this statement contributed to her electoral defeat in the Democratic primary later that year. McKinney returned to Congress in 2005, but her failure to present proper identification when entering a Congressional office building led to a physical altercation with a U.S. Capitol police officer in 2006, which resulted in a grand jury investigation and a formal apology on the floor of the House of Representatives. She was once again defeated in the Democratic primary, and she moved to California to pursue a doctorate at the University of California at Berkeley in 2007.

In 2008 at the Green Party convention in Chicago, McKinney was nominated as the party's candidate for president in that year's election. She selected journalist and community activist Rosa Clemente as her running mate. McKinney and Clemente received about 0.1 percent of the vote in the election.