He was an original member of Orson Welles's "Mercury Theatre Of The Air", which also included such actors as Joseph Cotten and Agnes Moorehead. He appeared in several films directed by Welles.
In the 1970s, Stewart had a "second career" as a voice actor, lending his dulcet, elegant vocal talents to many a TV commercial and occasionally narrating movie trailers.
Was a founding member of AFTRA (American Federation of Television and Radio Artists).