Leon served as a law clerk to the justices of the Massachusetts Superior Court from 1974 to 1975 and to Thomas F. Kelleher of the Rhode Island Supreme Court from 1975 to 1976. Leon was an attorney for the Immigration and Naturalization Service of the United States Department of Justice from 1976 to 1977 and a Special Assistant United States Attorney in the office of the United States Attorney for the Southern District of New York working in the Civil Division from 1977 to 1978.