Feinberg was born in Brockton, Massachusetts. He received a Bachelor of Arts from the University of Massachusetts-Amherst in 1967 and a J.D. degree from the New York University School of Law in 1970. He worked for five years as an administrative assistant and chief of staff for U.S. Senator Ted Kennedy and as a prosecutor for the U.S. Attorney General. Before founding his own firm The Feinberg Group (now the Law Offices of Kenneth Feinberg) in 1993, he was a founding partner at the Washington office of Kaye Scholer LLP.