Dehner was born in Staten Island, New York. He had an extensive career as a radio actor, appearing as a lead or supporting player in such series as Gunsmoke and Philip Marlowe. He starred as Paladin in the radio version of Have Gun – Will Travel, one of the few times a show began on television and then was later adapted for radio.   Written and directed by Antony Ellis, the short-lived series followed the adventures of journalist Kendall as he roamed the Western United States in search of stories for The Times.  Dehner appeared with Maudie Prickett in the 1953 episode “Bad Men of Marysville” of the syndicated Western television series The Adventures of Kit Carson, starring Bill Williams. He guest starred on the 1955–1956 NBC Western anthology series, Frontier and in the CBS Cold War drama, Crusader, starring Brian Keith. He played the sheriff-turned-outlaw Henry Plummer in an episode of the 1954-1955 syndicated Stories of the Century, starring Jim Davis as Matt Clark, the fictitious detective of the Southwest Railroad.  He delivered two memorable performances on the ABC/Warner Brothers western series, Maverick (1957) opposite James Garner in the episodes “Shady Deal at Sunny Acres” (“…if you can’t trust your banker, whom can you trust?”) and “Greenbacks, Unlimited.” He appeared as an enemy of the Sioux in the episode “The Broken Pledge” on another ABC/WB western series, Cheyenne, starring Clint Walker, with other guest stars Jean Byron, William Fawcett, Gary Vinson, Whit Bissell, and Frank DeKova.  Dehner played Sheriff Pat Garrett in Gore Vidal’s The Left Handed Gun, opposite Paul Newman as Billy the Kid. Dehner appeared in Scaramouche (1952) as Doutreval of Dijon, and he played the non-singing role of Mr. Bascombe, the mill owner and intended robbery victim, in the 1956 film version of Rodgers and Hammerstein’s Carousel.  He took the role of Colonel Tedesco in CBS’s “The Killers of Mussolini” on Playhouse 90, an original drama by A. E. Hotchner. In 1957, Dehner was cast in the film Texas Rangers with Gale Storm, and in the CBS western series, Gunsmoke, in the episode “Crack Up”. He guest starred on the syndicated series, Sheriff of Cochise, John Bromfield’s 1950s crime drama with a western setting.  In “Twelve Guns” (November 1, 1958) on NBC’s Cimarron City western series, Dehner played a prosperous area rancher, John Hartman, Sr., whose outlaw son, John, Jr., portrayed by Nick Adams, joins a gang of twelve that demands $50,000 from the citizens of Cimarron City. Previous towns have paid the ransom to be rid of the gang, the leader of which is the notorious Tate Masters, played by Charles Cooper.  In 1958, Dehner appeared on Perry Mason as Army prosecutor Major Lewis in “The Case of the Sardonic Sergeant.”

In the 1959–1960 television season, Dehner was cast in four episodes of another ABC/WB western series, The Alaskans, with Roger Moore as Silky Harris and Dorothy Provine as Rocky Shaw. He played a character “Cornish” in two episodes and “Soapy Smith” in two others. From 1960 to 1962, he was cast as Duke Williams in 27 episodes of the ABC/WB crime drama, The Roaring 20s, again with Dorothy Provine, this time in the role of Pinky Pinkham, and with five other co-stars, Mike Road, James Flavin, Gary Vinson, Donald May, and Rex Reason.  In 1960, Dehner was cast as Major Randolph, with James Coburn and Roger Perry, in the episode “Friend of the Family” on the CBS western, The Texan, starring Rory Calhoun. In 1960 and 1961, he was cast as John Sims in the episodes “The Scalp Hunter” and “Jerkwater” on the ABC western series, The Rebel, starring Nick Adams.  Dehner appeared in three episodes of The Twilight Zone: as Captain Allenby in the 1959 episode “The Lonely”; a 1961 episode, “The Jungle”, as an engineer who receives an African curse; and “Mr. Garrity and the Graves” in the series’ fifth and final season. He guest starred in the episode “Three” of the syndicated crime drama The Brothers Brannagan, starring Stephen Dunne and Mark Roberts as well as playing Arvid Lacey in the Rawhide episode “Incident at Sulphur Creek” in 1960, and in 1961, as Jubal Wade in the Rawhide episode “Incident of the New Start”. He returned to Rawhide in 1963 as Capt. Francis Cabot in the episode “Incident of Judgement Day”. In 1960 he guest starred on the Jack Lord ABC rodeo drama series, Stoney Burke.  Dehner played an old World War I French general who assists the Americans during World War II in the episode “The General and the Sergeant” of ABC’s Combat! During this time, he also appeared on another World War II television drama, ABC’s The Gallant Men, in an episode entitled “A Moderately Quiet Sunday”, in which he portrays a disillusioned German captain who contrives to surrender himself to an American private.

Dehner appeared as different characters on a number of episodes of ABC’s The Rifleman, starring Chuck Connors. Dehner guest-starred in NBC’s The Wide Country, a 1962-1963 drama about rodeo performers starring Earl Holliman and Andrew Prine. On March 4, 1962, he appeared as legendary Sheriff Ben Wyatt on the ABC/WB western series, Lawman. In the episode entitled “The Long Gun”, Marshal Dan Troop (John Russell) is determined to prevent Wyatt from shooting two murderers to death with his rifle, instead of taking them alive for trial.  Late in 1962, Dehner guest starred as Dan Tabor in the episode “Echo of a Man” of the NBC western with a modern setting, Empire, starring Richard Egan as rancher Jim Redigo. Also that year, he played a mad scientist creating havoc about the submarine Seaview in the Voyage to the Bottom of the Sea episode “The Men Fish.”  In 1966, Dehner played the assassin, “Iron Man” Torres, in the “Night of the Steel Assassin” episode of CBS’s The Wild Wild West, starring Robert Conrad.  Dehner appeared in comedy, having been cast in three episodes of the CBS military sitcom, Hogan’s Heroes, as General von Platzen. He guest starred on the Walter Brennan ABC sitcom, The Real McCoys. He guest starred on an episode of CBS’s The Andy Griffith Show as Colonel Harvey, purveyor of a magic elixir that casts a spell over Aunt Bee (Frances Bavier).  Dehner appeared on Jack Palance’s ABC circus drama, The Greatest Show on Earth and on the CBS sitcom/drama Glynis, starring Glynis Johns and Keith Andes. He was featured in 1965 in ABC’s F Troop episode “Honest Injun” as a dishonest traveling patent medicine salesman. In 1966, he guest starred in the episode “Power of Fear” of Barry Sullivan’s NBC western series, The Road West and played the recurring role of Morgan Starr on The Virginian. In 1970, he appeared in The Cheyenne Social Club with James Stewart and Henry Fonda. He also guest-starred in an episode of The Silent Force that year.

From 1971 to 1973, Dehner was Cy Bennett, the character Doris Martin’s overbearing boss on CBS’s The Doris Day Show.  He played a sympathetic judge in the courtroom thriller Jagged Edge. He appeared on NBC’s Columbo episodes Swan Song (1974) and Last Salute to the Commodore (1976), playing the title role in the latter, the only Columbo episode in which the killer is unknown from the beginning.  In 1977 he played in an episode of The Rockford Files entitled “There’s One in Every Port” where he played the parts of Judge Lyman and Inspector DeMartonis. In 1983, he starred in the short-lived NBC prime time soap opera Bare Essence in the role of Hadden Marshall.  Dehner also portrayed a number of historical figures, including the previously mentioned portrayal of Pat Garrett in the 1957 film, The Left Handed Gun; Jean Lafitte in the 1964 episode “The Gentleman from New Orleans” of NBC’s Bonanza; Thomas Jefferson in the 1964 episode “Plague” of the anthology series, The Great Adventure; Dean Acheson in the 1974 TV-movie The Missiles of October; Lafayette C. Baker in the 1977 film The Lincoln Conspiracy; John Muir in the 1979 TV movie Guardian of the Wilderness (also known as Mountain Man); Henry Luce in the 1983 film The Right Stuff, about the Mercury astronauts, and Admiral Ernest J. King in the 1988 television miniseries War and Remembrance.  Dehner died of emphysema and diabetes at the age of 76 in Santa Barbara, California. His interment is at Carpinteria Cemetery in Carpinteria, California.