After two years in the United States Army making training films, he returned to Warner Brothers, where he was employed in the publicity department and then as a second assistant director.

(1956), as tabloid reporter Jess Blanchar, Rawhide Trail (1957), as Farley Durand, and Revolt at Fort Laramie (1957), as Lt. Chick Waller.

In 1959, Knapp was cast in the lead as Gil Reardon in the film Gunmen from Laredo and he played the coroner "Morty" in Roman Polanski's Chinatown.

His other film roles included Hot Car Girl as Detective Lieutenant Ryan (1958), The Threat as Steve Keenan (1960), and The Stoolie as "Big Daddy" (1972).

Knapp was cast as Tom Dixon, a former outlaw trying to change his life and marry his sweetheart in the 1960 episode, "The Devil's Due", on the syndicated anthology series, Death Valley Days, hosted by Stanley Andrews.

Several months earlier, he had played Andy Watson in the episode "Underwater Narcotics" of a similar series, the syndicated Sea Hunt starring Lloyd Bridges.

Twice Knapp appeared on the following: The Public Defender, Navy Log, The Silent Service, Hawaiian Eye, Bonanza, The Rat Patrol, Run for Your Life, M Squad, Boots and Saddles, Lock-Up, The Invaders, and Cheyenne.

Knapp's last role was in 1975 in the Eddie Albert/Robert Wagner CBS drama Switch, in which he played Lieutenant Stone in the unusually titled episode "Gaffing the Skim".