Born in Chicago, Illinois, to professional ballroom dancers, Haakon Olaf Hagen and the former Marvel Lucile Wadsworth. When Haakon Hagen deserted his family, young Hagen was reared by his mother, grandmother, and aunts. As a 15-year-old, he relocated to Portland, Oregon, where one of his aunts had taken a teaching job. His family returned to Chicago, but he spent a year in law school at the University of California, Los Angeles, after having attended Oregon State University in Corvallis and the University of Southern California in Los Angeles, California, from which he had received a degree in International Relations. Subsequently employed by the U.S. State Department in West Germany (since 1990 Germany), he then served a two-year stint in the United States Navy. For a time he taught ballroom dancing, the specialty of his parents, for the Arthur Murray Company. Then, at the age of twenty-seven, he tried acting. He was spotted in a production of Eugene O’Neill’s Desire Under the Elms and given a guest-starring role on the classic 1950s police series Dragnet, starring Jack Webb.

Hagen began to work steadily in television and film. His first regular role on a series was in 1958 in the CBS cult western Yancy Derringer, starring Jock Mahoney in the title role. Hagen played John Colton, the city administrator of New Orleans, c. 1868. At the beginning of each episode, Colton asks Derringer to halt some threat facing the city; at the end of each segment, he arrests Derringer for breaking the law to solve the crisis.  On April 29, 1962, Hagen was cast as the lead guest star in the episode “Cort” of the ABC-WB western series, Lawman, with John Russell and Peter Brown. In the story line, Cort Evers, who is much younger than he appears, seeks revenge against his brother Mitch (Harry Carey, Jr.), whom he mistakenly blames for betraying six Union Army prisoners from their hometown during the American Civil War. Mitch is compelled to contront Cort in a shootout during which he explains that it had been Cort himself, under the influence of a fever, who betrayed the prisoners. Cort faints to the ground as he remembers the startling truth of the betrayal.  From 1969 to 1970, Hagen appeared as Inspector Dobbs Kobick in nine episodes of Land of the Giants.

Hagen guest starred seven times on Gunsmoke, six times on The Big Valley, five times each on Bonanza, Laramie, and Have Gun – Will Travel, four appearances on Mannix and The Time Tunnel, and three appearances on Perry Mason, two of them in 1965: as murderer Jacob Leonard in “The Case of the Gambling Lady,” and Samuel Carleton in “The Case of the Fugitive Fraulein.”  Other appearances included Bat Masterson, Riverboat, Wagon Train, Outlaws, Straightaway, GE True, Voyage to the Bottom of the Sea, The Twilight Zone, in the episode “You Drive”, Daniel Boone, Blue Light, Mission: Impossible, Rawhide, 77 Sunset Strip, M*A*S*H, The Rifleman, Lancer, The Virginian, The Guns of Will Sonnett, The Cowboys, Lost in Space (as the alien in the episode “His Majesty Smith”), The Silent Force, Sara, Simon and Simon, and Knots Landing.  He considered his big break to be the role of a Confederate soldier who kills James Stewart’s son and daughter-in-law in the 1965 film Shenandoah. His most famous role was one of his most pleasant, as kindly Doc Baker on Michael Landon’s Little House on the Prairie. He played the part from 1974 to 1983, as well as in a one-man show, A Playful Dose of Prairie Wisdom.  In 1992, he moved to Grants Pass in Josephine County in southwestern Oregon, and continued his acting career. In 2004, he was diagnosed with esophageal cancer.  He was married to actress Susanne Cramer until her death in 1969. At the time of his death, Hagen left a widow, Jan, his fourth wife whom he met in 1993, and a son, Kristopher.