After several more films Kelley moved to New York, where he worked onstage and in early TV anthology dramas. He returned to Hollywood and was helped by a friend who was writing for TV's "You Are There". Kelley also resumed his feature career in modest roles in Samuel Fuller's "House of Bamboo" (1955) and "Tension at Table Rock" (1956). The latter set a pattern for Kelley, whose slight Southern drawl and lean, weathered look suited him for roles as townsfolk, prairie ranchers or lesser villains in Westerns including "Warlock" (1959), "Town Tamer" (1965) and "Gunfight at the OK Corral" (1956), as one of the Earp brothers.
Starring for writer-producer Gene Roddenberry on an NBC-TV crime drama pilot, "333 Montgomery" (1960), didn't seem anything special, especially when the resulting series, "Sam Benedict", cast Edmond O'Brien in the lead. Roddenberry remembered Kelley, though, and on the original "Star Trek" (1966-69) series, the actor found a showcase part. "I'm just a country doctor" and "He's dead, Jim" became trademarks for the argumentative, old-fashioned and humanistic McCoy, a marvelous foil for the more logical Spock and more macho Kirk.
Kelley's only subsequent non-"Trek" feature was the laughable horror film, "Night of the Lepus" (1972)--also co-starring Shatner--but he did some TV guest spots and reprised his greatest role in a TV cartoon (1973-75) and in the six immensely successful "Star Trek" feature films from 1979 to 1991--he also helped christen "Star Trek: The Next Generation" by cameoing in the first two-hour episode as an aged "Bones" as a favor to his longtime friend Roddenberry, becoming the first--but far from the last--"Trek" actor to reprise their role for one of the franchise spin-offs. While Kelley's laid-back charm and twinkling eyes frequently steal scenes from his co-stars, he emerged as the one member of the original cast who was universally admired by the frequently battling actors. Kelley also repeatedly took great pride whenever he encountered fans of the series who were inspired to become physicians by his portrayal of McCoy. He was the first of the original cast to pass away, in 1999.