As an adult, his first job in films was as a co-writer of the 1939 Ronald Reagan movie Code of the Secret Service.

Throughout the 1950s and 1960s, Riesner worked primarily in television, including writing for Rawhide and the "Tourist Attraction" episode of The Outer Limits, although he occasionally contributed to feature films like The Helen Morgan Story.

Riesner helped pen the screenplays for two Eastwood films in 1971, Play Misty for Me and the original Dirty Harry.

In 1973 he provided an uncredited rewrite for High Plains Drifter, and in 1976, he was one of the writers to draft The Enforcer, the third Dirty Harry thriller.

In 1979, he wrote an early draft screenplay for The Godfather Part III, but his script was discarded when Francis Ford Coppola and Mario Puzo agreed to collaborate on a third entry in the series.