First met his wife, the former Barbara Long, through Roddy McDowall and reconnected later at the Laguna Playhouse while both were appearing in the play "French Without Tears" in 1948. A year later they married and had one daughter, Janet. He subsequently appeared with his brother-in-law Richard Long in Cult of the Cobra (1955).
He produced, directed and starred in A Yank in Viet-Nam (1964), a film that could have had tragic consequences. It was an on-location anti-Viet Cong picture that had the Viet Cong putting a price on his head during the shoot. The picture was important in that it was the first filmed during the war and while under fire.
His dentist father, Laurence Thompson, was a decorated WWI medic who received shrapnel injuries during his tour of duty. Decades later, a small fragment dislodged and caused a fatal stroke. He was in his late 60s. His mother, Pauline, was a one-time concert singer. His parents met while she was entertaining WWI troops.
Life-long friend of actress Betty Lynn, best known as Thelma Lou on "The Andy Griffith Show" (1960). Both graduated from University High School in Los Angeles.
Unlike some sources indicate, Marshall never had any intentions on becoming a minister -- the story was promoted by MGM's publicity department; however, he was a pre-med student at Occidental College at one time before focusing on drama.