Harmon received her law degree at Yale Law School, where she was articles editor for the Yale Law Journal and the Yale Journal of Law and the Humanities. Before law school, as a British Marshall Scholar, she earned an M.Sc. in political theory and an M.Sc. in political sociology, both with distinction, from the London School of Economics. After law school, she clerked for Judge Guido Calabresi of the U.S. Court of Appeals for the Second Circuit and Justice Stephen Breyer of the U.S. Supreme Court.  

From 1998 to 2006, Harmon served as a prosecutor at the U.S. Department of Justice.  After a brief stint at the United States Attorney’s Office in the Eastern District of Virginia, Harmon worked in the Civil Rights Division, Criminal Section, prosecuting hate crimes and official misconduct cases, many of which involved excessive force or sexual abuse by police officers.  She left the Justice Department to join the law faculty as an associate professor of law in the fall of 2006.