Kelley had previously been best known as heavies in western films, but his most famous screen role - in the original series and the first six Star Trek films - was fitting because medicine was in the actor's family. His uncle, an eminent physician, delivered him at his parents' home in Atlanta, Georgia, in 1920 and the young Kelley wanted to be a doctor, but the effects of the Depression meant that his parents could not afford to send him to medical school.
Kelley's father was a Baptist minister and it was the experience of singing in a church choir that made the youngster want to perform. He eventually sang on the Atlanta radio station WSB, which led to an engagement with Lew Forbes and his orchestra at the Paramount Theater.
The chance to act came after staying with an uncle in Long Beach, California. A two-week break became a permanent arrangement and Kelley was working as a lift attendant when director Rohn Hawke spot- ted the 17-year-old and worked with him to smooth out his Georgia accent before casting him in a stage play with the Long Beach Theater Group.
Kelley then made his first feature film appearance, in the chorus of MGM's New Moon (1940), featuring Oscar Hammerstein and Sigmund Romberg's score and a cast led by Jeanette MacDonald and Nelson Eddy, cinema's most successful singing duo in musicals of the time. Kelley 's big break appeared to have arrived when, after 13 auditions, he was told he had secured the role of the baby-faced killer in This Gun for Hire (1942), but it was then handed to Alan Ladd.
Further work with the Long Beach Theater Group was interrupted by America's involvement in the Second World War. Kelley served in the Army Air Corps in New Mexico, before being transferred to Culver City to act in a Navy training film, Time to Kill (1945). Spotted in it by a Paramount talent scout, he was offered a three-year film contract and eventually made his debut as Vince Grayson in the well received chiller Fear in the Night (1947). He went on to appear in a string of mostly forgotten pictures. He also spent three years on stage and television in New York.
Back in Hollywood, he became typecast as heavies in films - particularly westerns - after acting Ike Clanton in a "Gunfight at the OK Corral" episode of the television series You Are There (1955), which featured the stories of famous historical figures. He switched sides to play Morgan Earp, brother of Wyatt, in Gunfight at the OK Corral (1956), starring Burt Lancaster and Kirk Douglas as Wyatt Earp and his sidekick "Doc" Halliday blasting to smithereens the three cattle- rustling cowboys who threatened them in the streets of Tombstone, Arizona.
Kelley was also seen in the Elizabeth Taylor-Montgomery Clift Civil War drama Raintree County (1957) and alongside Robert Taylor and Richard Widmark in The Law and Jake Wade (1958). He appeared on televison in episodes of classic western series such as The Lone Ranger (1949), Gunsmoke (1955), 77 Sunset Strip (1958), Rawhide (1959), Bonanza (1959, 1961, 1962), Laramie (1959) and The Virginian (1962), as well as in The Fugitive (1963) and Ironside (1967).
But it was the chance to act in a new science-fiction series, in 1966, that brought Kelley lasting fame. Star Trek (1966-69) was Gene Roddenberry's vision of a future without wars and famine. He had written the western series Have Gun, Will Travel and planned Star Trek as a space-age "horse- opera". Instead of the Old West, the setting was the "final frontier" of space in the 23rd century.
Dr Leonard "Bones" McCoy was conceived as a Southern gentleman ship's medical officer aboard the USS Enterprise, whose five-year mission was to seek out new life forms and civilisations. There was a special bond between McCoy and Captain James T. Kirk (William Shatner), and their relationship with Mr Spock (Leonard Nimoy) was central to the stories, which were sometimes used to pass social comment on issues such as the Vietnam War, racism and totalitarianism. Kelley was even able to reprise his Gunfight at the OK Corral screen appearances in a 1968 Star Trek episode entitled "Spectre of the Gun", this time back on the side of the Clantons.
However, the programme was not an immediate hit and it was lucky to be commissioned for a second series. After three years, Star Trek was taken off the air, but its 79 episodes gained new-found popularity in repeat runs after the American Moon landing. It has continued to be repeated ever since and remains the most popular cult television series of all time.
As a result, the cast were reunited for Star Trek: the motion picture (1979), trying to combat a lethal force field heading for Earth. Kelley, who had been promoted to Commander McCoy, and the other original cast members all appeared in five sequels: Star Trek II: the wrath of Khan (1982), Star Trek III: the search for Spock (1984), The Voyage Home: Star Trek IV (1986), Star Trek V: the final frontier (1989) and Star Trek VI: the undiscovered country (1991). The programme itself spawned three sequels, The Next Generation, Voyager and Deep Space Nine, with new stories and different actors, although Kelley voiced his character in the 1973 Star Trek animated series and reappeared as Admiral Leonard McCoy in a 1987 episode of The Next Generation.
He turned down the opportunity to appear in the seventh Star Trek film, Star Trek: generations (1994), going into semi-retirement after Star Trek VI, but he continued to delight fans by attending conventions and appeared as himself in last year's documentary film Trekkies, about how the programme had affected the lives of devotees. DeForest Kelley also provided one of the voices in the Disney animated sequel The Brave Little Toaster Goes to Mars (1998), which failed to gain a theatrical release.
Jackson DeForest Kelley, actor: born Atlanta, Georgia 20 January 1920; married 1945 Carolyn Dowling; died Woodland Hills, California 11 June 1999.