His film debut came in 1948 with "The Naked City," and Gregory spent the next two decades turning in memorable supporting roles, mostly in actioners, crime dramas and Westerns. His commanding sneer and razor-sharp voice gained him good roles in Michael Curtiz's crime drama "The Scarlet Hour" (1956), the rebellious youth film "The Young Stranger" (1957), the William Wyler oater "The Big Country" (1958), the political thriller "The Manchurian Candidate" (as a senator, 1962), "P.T. 109" (1963, as Kennedy's commander), the crime spoof "The Silencers" (1966), "Clambake" (1967, as Elvis Presley's dad), "Beneath the Planet of the Apes" (1970, unrecognizable beneath a rubber mask) and the Barbra Streisand-Ryan O'Neal boxing romance "The Main Event" (1979). In all, he made some 40 films.
Gregory broke into TV in the early 50s, with such anthologies as "Eye Witness", "Star Tonight", "Pursuit", "Buick Electra Playhouse" and the 15-minute "Short, Short Drama". He had his own series--as a Prohibition agent--in the hard-boiled "The Lawless Years" (NBC, 1959-61). Gregory made his first TV-movie in 1961, the comedy "Happily Ever After" (NBC), and went on to become a reliable presence as a supporting player in more than a dozen more. He was in the pilot for "Hawaii Five-O" (CBS, 1968) as well as "The Flim-Flam Man" (NBC, 1969), "A Very Missing Person" (ABC, 1972), "Francis Gary Powers: The True Story of the U-2 Spy Incident" (NBC, 1976), the syndicated miniseries "The Bastard" (1978), "The Great American Traffic Jam" (NBC, 1980) and "Wait Till Your Mother Gets Home" (NBC, 1983).
But it is as a series regular that Gregory is best-remembered. After "The Lawless Years", he turned up on a semi-regular basis in three Westerns: "The Big Valley" (ABC, 1965-69), as President Grant in "The Wild, Wild West" (CBS, 1965-70), and as the gruff Major Duncan in the comic "F Troop" (ABC, 1965-67). He played Don Hollinger's boss in "That Girl" (ABC, 1966-71) and Police Inspector Luger in "Barney Miller" (ABC, 1975-82). That last role led to another starring series, the short-lived "Detective School" (ABC, 1979). He guested on shows into the 1980s, including "Mr. Belvedere" (1986) and "Dolly" (1988).