Popwell was featured on many television series, but is perhaps best known for his appearances in films opposite Clint Eastwood, whom he acted with in five films, starting with Coogan's Bluff and in the first four films of the Dirty Harry series.
Popwell blev castet til en lang række tv-serier, men er formentlig bedst kendt for sin optræden I fem film med Clint Eastwood, første gang i Coogan's Bluff og senere i de første fire film i serien om Dirty Harry.