At a time not far removed from segregation, Clarence Cooper began practicing law as a Legal Aid Society lawyer. He later began working for Fulton County District Attorney Lewis Slayton before being selected to fill a seat on the Municipal Court. He became the first African-American to be elected to the Fulton County Superior Court in 1980. Less than two years later he presided over the trial of Wayne Williams, accused murderer in the Atlanta Child Murder Case.

He was appointed to the federal bench because of his work in both Fulton County and on the Georgia Court of Appeals.