Also in 1955, Platt appeared in Rebel Without a Cause starring James Dean, Natalie Wood and Sal Mineo.In 1957 he had a supporting role in “Designing Woman” with Gregory Peck and Lauren Bacall.

Platt also appeared in episodes of the original Perry Mason ("The Case of the Slandered Submarine" and "The Case of the Larcenous Lady"); 77 Sunset Strip, Trackdown, State Trooper (in the episode "Who Killed Doc Robbins"), Men into Space ("From Another World"), Ripcord ("Million Dollar Drop"), Gunsmoke, Bonanza, One Step Beyond, The Rifleman (S5 E7 "The Assailants" as Sen. Bordon 1962), Rawhide, Whispering Smith, The Outer Limits ("The Man with the Power" and "Keeper of the Purple Twilight"), The Dick Van Dyke Show ("A Nice Friendly Game Of Cards"), Voyage to the Bottom of the Sea, Bat Masterson (in the 1959 episode "Incident in Leadville"), Tales of Wells Fargo, The Twilight Zone, and Wagon Train Platt was widely known for his role as The Chief in the espionage parody television series Get Smart (1965–1970).

Platt appeared as Sotto Voce in the 1969 KCET television reading of Norman Corwin's 1938 radio play The Plot to Overthrow Christmas.

His crew shot the production with electronic TV cameras and portable video tape recorders, then had the images transferred to film for theatrical release.

[5] Initial reports indicated the cause of death was a heart attack, but Platt's son later said that his father died from suicide after a long struggle with untreated depression.