Kennedy was born in Florida and studied mathematics at several universities. From 1961 he was professor of mathematics, with research interest in the history of mathematics, at Providence College (Rhode Island), He spent three sabbatical years doing research in Italy and Germany.
In 1986 Kennedy moved to San Francisco, where he continued his historical research, now in the beginnings of the gay movement in Germany. Since 2003 he has been in a home for assisted living in Concord, California. He has over 200 publications in several languages, from an analysis of the mathematical manuscripts of Karl Marx and a revelation of Marx's homophobia, to theoretical genetics and a proof of the impossibility of an organism that requires more than two sexes in order to reproduce. In addition, Dr. Kennedy has written biographies of the Italian mathematician Giuseppe Peano and the German homosexual emancipationist/theorist Karl Heinrich Ulrichs, and has edited the collected writings of Ulrichs. His translations of the boy-love novels of the German anarchist writer John Henry Mackay and his investigations of the writings of Mackay have helped establish Mackay's place in the gay canon.
Kennedy married his long time companion Don Endy in 2014.
- John Henry Mackay (Sagitta), Anarchist der Liebe, biography (2007).
- Karl Heinrich Ulrichs, Pioneer of the Modern Gay Movement, (2002).
- PEANO: Life and Works of Giuseppe Peano.
- Sex & Math in the Harvard Yard: The Memoirs of James Mills Peirce, novel.
- Homosexuality and Male Bonding in Pre-Nazi Germany: The Youth Movement, the Gay Movement and Male Bonding Before Hitler’s Rise: Originalübersetzungen aus Der Eigene, (1991).
- Life’s Little Loafer, short story.
- From My Life (2009), autobiography by Kennedy.
- A Touch of Royalty: Gay Author James Barr, essay.
- Eight Mathematical Biographies, collections of short biographies.
- The Ideal Gay Man: The Story of Der Kreis, (1999).
- Negation of the Negation: Karl Marx and Differential Calculus.
- In Memoriam: Five Gay Obituaries over Glenn Hogan, Mario Mieli, Roger Austen, Peter Schult and Robert Turner.
- John Henry Mackay, Die letzte Pflicht & Albert Schnells Untergang, (2007).