(written from a Production point of view)
Mark Dempsey (29 January 1936 – 4 April 1994; age 58) was an actor who played the Air Force captain in the Star Trek: The Original Series episode "Tomorrow is Yesterday". He filmed his scene on Monday 5 December 1966 at Desilu Stage 10.
Dempsey was born as Herbert L. Dempsey in West Hollywood, California and played regular roles in stage plays beside his on-screen appearances.
Among his acting resume are the films Valley of the Dragons (1961, with Gil Perkins), Palm Springs Weekend (1963, with Bill Mumy, Nick Dimitri, and Judi Sherven), Oh, Calcutta! (1972), The Prodigal (1983), and To Cross the Rubicon (1991, with Billy Burke). He also guest starred in television series such as G.E. True (1962), Hawaiian Eye (1963, with Joan Marshall), 77 Sunset Strip (1963), Perry Mason (1964, with John Harmon), and Bergerac (1989).
He died on 4 April 1994 in Seattle at the age of 58.