Duff was born on September 15, 1930, in Dallas, Texas as the third of ten children.
He was an assistant to Chief Executive Officer of Banker's Life and Casualty from 1962 to 1967.
Duff was a Vice President and general counsel of R. H. Gore Co. from 1968 to 1969 and was in private practice from 1965 until 1976 in Chicago, Illinois.
He was also a Judge of the Circuit Court of Cook County, Criminal Division from 1976 to 1979, and then at Circuit Court of Cook County, Law Jury Division from 1979 to 1985.
Duff assumed senior status due to a certified disability on October 30, 1996.