Popwell had already at the age of 16 years, made his Broadway debut in The Pirate. He played in several Broadway productions until the mid-1960s; partly as a dancer, partly as an actor. He then began his career as a film and television actor first with guest roles in television series. In 1968, he first stood next to Clint Eastwood in front of the camera, as he starred in Coogan's Bluff great a supporting role. As a result, he stepped up to the last part of the series on all Dirty Harry movies. In the first part of the film series Popwell will not appear in the credits; However, he is known as the bank robber, the Clint Eastwood at the beginning of the film at gunpoint with the words Do you feel lucky punk? challenges, some of the most famous scene of the series. In the next two parts of the series, he was seen as a gangster, before he comes back in Dirty Harry played his partner. Gained fame Popwell in the 1970s for his role of Matthew Johnson in the blaxploitation films A Case for Cleopatra Jones and Cleopatra Jones against the dragon lady.
In addition to his film career Popwell was seen between the 1960s and 1980s in different series formats, including in guest roles in The Streets of San Francisco, Simon & Simon and The A-Team. He also appeared in the miniseries Roots - The Next Generations on.