During his studies at Harvard Law School, Lader was an Assistant Professor of Political Science at Newton College of the Sacred Heart (subsequently merged with Boston College) and a teaching assistant to Harvard Law Professor Paul Freund and Harvard Political Philosophy Professor Louis Hartz. After graduation, he was a law clerk to the late Judge Paul Roney, Chief Judge of the U.S. Court of Appeals for the 11th Circuit (formerly Fifth Circuit) and was associated with the New York law firm of Sullivan & Cromwell. He served in the U.S. Army (JAG) Reserves from 1969 to 1975.