In 1960, Lockhart made his Broadway debut, playing a gang leader in The Cool World (a dramatization of Warren Miller's novel of the same name), which closed after just two performances. Lockhart then travelled to Italy and formed his own theatre company in which he both acted and directed, before moving to West Germany and then England, where he landed various roles on British television and small roles in films such as 1966's Talking to a Stranger, where he played Judi Dench's character's husband, and 1968's A Dandy in Aspic and Salt and Pepper.
Lockhart's first notable screen role was in Joanna, a 1968 film about an interracial romance, set in London. Joanna was directed by Michael Sarne, who subsequently cast Lockhart in the notorious Myra Breckinridge. Lockhart's first lead role in a film was in Halls of Anger (1970), playing a former basketball star who becomes vice-principal of an Anerican inner-city high school to which 60 white students are being moved. An article in The New York Times that year described Lockhart as having "matinee-idol looks" with "chiseled-out-of-marble features" and "skin the color of brown velvet". He also starred in Cotton Comes to Harlem (1970, based on the Chester Himes novel of the same name) as the Reverend Deke O'Malley. In 1974, Lockhart became an actor-in-residence at the Royal Shakespeare Company in Stratford-upon-Avon. In the 1980s, he was a guest star for seven episodes in the American prime-time soap opera Dynasty, playing Jonathan Lake. He is familiar to horror film fans after his performance as the millionaire big-game hunter in The Beast Must Die (1974), and for his small appearances in two films by David Lynch, Wild at Heart (1990) and Twin Peaks: Fire Walk with Me (1992). Also in 1990, Lockhart made an appearance as the Jamaican drug lord King Willie in Predator 2.
