After law school, Garrett clerked for U.S. Supreme Court Justice Thurgood Marshall and for Judge Stephen F. Williams on the U.S. Court of Appeals for the District of Columbia Circuit. She also served as a legal advisor at the Iran–United States Claims Tribunal at The Hague and as legislative director and tax and budget counsel for Senator David L. Boren of Oklahoma. Boren said about Garrett, "If I were to count on the fingers of one hand the people I’ve known with the most remarkable intellect, she would be on that list."