Originally from Otterbein near Lafayette, Indiana, Kennedy graduated from DePauw University in Greencastle, Indiana. He studied acting under Sanford Meisner at the Neighborhood Playhouse in Manhattan.

Kennedy's first acting role, though uncredited was at the age of thirty-three as Yip Ryan, a United States Army pilot in the 1955 film, The Court-Martial of Billy Mitchell. The film also starred Gary Cooper as Billy Mitchell and Dayton Lummis as General Douglas MacArthur. In 1956, he appeared in two anthology series: as Benton in the episode "In a Small Motel" of the Chevron Hall of Stars and in "Outpost" of the H.J. Heinz Company's Studio 57. In 1957, he appeared in two syndicated series about the United States Navy: Men of Annapolis ("Mister Number Five") and The Silent Service ("The Spearfish Delivers"). That same year, he appeared in another anthology, CBS's Dick Powell's Zane Grey Theater as Adam Dempster in the episode "A Man on the Run".

Cast in The Californians, Kennedy appeared in such episodes as "Skeleton in the Closet", "Pipeline", "The Foundling", "Second Trial", "The Inner Circle", "The Golden Bride", "Murietta", "Shanghai Queen", "Bridal Bouquet", and "Golden Grapes", his final segment on the program on June 17, 1958. His co-stars were Sean McClory and Herbert Rudley.

Between 1955 and 1958, Kennedy appeared in four different roles in the CBS anthology series Schlitz Playhouse of Stars: as Charlie in "The Careless Cadet", as George in "Always the Best Man", as Johnnie in "The Blue Hotel", and as Steve Elliot in "The Trouble with Ruth". He appeared too in 1959 with Mary Astor, Suzanne Pleshette, and Inger Stevens in the episode "Dairy of a Nurse" of CBS's Playhouse 90.

He appeared twice on the half-hour version of CBS's Gunsmoke: as Andy Travis in "Kite's Reward" (1955) and as Bert Wells in "Gentleman's Disagreement" (1960). In 1962, he appeared as Sam Hagen in the episode "Stopover in Paradise" of another CBS western, Frontier Circus, starring Chill Wills. In 1965, he appeared as Brock Hayden in the NBC soap opera The Doctors. His final screen role was an uncredited part in the 1980 television movie, Act of Love, starring Ron Howard and Robert Foxworth and directed by Jud Taylor.

After his acting career ended, Kennedy adapted a novel into a screenplay for Stanley Kramer's 1977 film, The Domino Principle, which stars Gene Hackman as a convict who becomes an assassin. Kennedy wrote nine other screenplays, including Raise the Titanic, The Dove and Barlow's Kingdom. He penned twenty novels, including The Killing Season (1967), Maggie D. (1973), Just Like Humphrey Bogart (1978), The Fires of Summer (1987) and Somebody's Fool (1993). Kennedy's oil paintings and watercolors have been displayed in the United States and Europe. In 1952, he was hailed in Paris as that city's most influential American painter.

Kennedy was married to the former Susan Adams. The couple had two sons, Regan Kennedy, living in New York City at the time of his father's death, and Jack Kennedy, then of Santa Barbara, California; a daughter, Anne Kennedy Stromsted of Norway, and a grandson. According to his former agent, Kennedy died at the age of seventy-five of a heart attack at home in Kent in Litchfield County in western Connecticut.