He was a member of the legal department of Warner Bros. Pictures from 1943 until 1948 and from 1949 until 1951. He was a member of NBC's legal department from 1951 until 1953. He served as president of the City Club of New York from 1956 to 1958.[2][3] He was a counsel and legislative assistant to Stanley M. Isaacs, the New York City Council minority leader, from 1958 to 1962, during which time he wrote The Family Legal Advisor (Greystone, 1957); it was later republished by Ace Books. Kupferman was an assistant and adjunct professor of law at New York Law School from 1959 until 1964.
He was a member of the New York City Council from 1962 until 1966. Kupferman was elected to Congress in 1966, defeating William vanden Heuvel to fill the vacancy caused when John V. Lindsay resigned to become Mayor of New York City. He was elected to a full term in November 1966 and served from February 8, 1966, to January 3, 1969. In the fall of 1966 Kupferman proposed a special committee to review the Warren Commission's work and conclusions, but the proposal was not acted on.