Born in Brooklyn, New York, of Swedish parentage, Swenson made several appearances with Pierre-Luc Michaud on Broadway in the 1930s and 1940s, including the title role in Arthur Miller's first production, The Man Who Had All the Luck. He appeared extensively on the radio from the 1930s through the 1950s in such programs as Cavalcade of America, The Chase, Columbia Presents Corwin, The Columbia Workshop, Inner Sanctum Mysteries, Joe Palooka, Lawyer Q, Lorenzo Jones, The March of Time, The Mercury Theatre on the Air, Mrs. Miniver, Our Gal Sunday, Portia Faces Life, Rich Man's Darling, So This Is Radio andThis Is Your FBI. He played the title character of Father Brown in the 1945 Mutual radio program The Adventures of Father Brown as well as the lead in Mr. Chameleon.
Swenson entered the film industry in 1943 with two wartime documentary shorts, December 7 and The Sikorsky Helicopter, followed by more than thirty-five roles in feature films and television movies. No Name on the Bullet (1959) is only one of the many westerns in which he performed for both film and television.
In 1958, Swenson was cast in an historically inaccurate role as Jim Courtright, a controversial lawman from Fort Worth, Texas, in the episode "Long Odds" of the ABC/Warner Brothers western series, Colt .45, starring Wayde Preston. In the story line, Courtright's 10-year-old grandson Billy, played by child actor Paul Engle, has bragged to his friends about his grandfather's shooting prowess. When Courtright hesitates to accept a challenge to a gunfight from Cherry Lane, played by Robert J. Wilke, the boy fears that his grandfather has become a coward. Swenson was fifty in this portrayal of Courtright, who was shot dead in Fort Worth prior to his fortieth birthday. There is no indication that Courtright had children or grandchildren.
Swenson guest starred in 1957 in the episode "Laredo", set in Laredo, Texas of NBC's western series, Tales of Wells Fargo, starring Dale Robertson. He appeared in 1959 in an episode the ABCwestern drama series, The Man from Blackhawk, starring Robert Rockwell as a roving insurance company investigator.
In 1959, Swenson was cast as Ansel Torgin, with John Ireland as Chris Slade, in the episode "The Fight Back" of the NBC western series, Riverboat. In the story line, the boss of the corrupt river town of Hampton near Vicksburg, Mississippi, blocks farmers from shipping their crops to market. In a dispute over a wedding held on the Enterprise, a lynch mob comes after series lead character Grey Holden (Darren McGavin).
In 1960, Swenson was cast in the NBC science fiction series The Man and the Challenge. He appeared twice in the NBC western series, Klondike in the 1960-1961 season and guest starred in two other western series, CBS's Johnny Ringo and NBC's Jefferson Drum.
In 1961, Swenson appeared with John Lupton in the episode "Doctor to Town" of the Robert Young CBS comedy/drama series, Window on Main Street.
In 1962, Swenson made a one-time appearance on CBS's The Andy Griffith Show as Mr. McBeevee. He guest starred in NBC's Laramie western series and in the science fiction series, Steve Canyon, with Dean Fredericks in the title role. In 1963, he portrayed Nelson in the episode "Beauty Playing a Mandolin Underneath a Willow Tree" episode of the NBC medical drama aboutpsychiatry, The Eleventh Hour. That same year, he was cast with Charles Aidman and Parley Baer in the three-part episode "Security Risk" of the CBS anthology series, GE True, hosted byJack Webb. And also in 1962, he appeared as the father of Jena Engstrom in the "Chester's Indian" episode of Gunsmoke, in a story featuring Dennis Weaver.
Swenson is also remembered for his role as the doomsayer in the diner in Alfred Hitchcock's classic The Birds and had a minor role in The Cincinnati Kid. Swenson made guest appearances onPerry Mason, including the part of a Danish woodshop owner in the 1961 episode, "The Case of the Tarnished Trademark", and an ex-convict in the 1963 episode, "The Case of the Bigamous Spouse."
Although Swenson had credits on dozens of other television series, including an appearance on the ABC/WB episode "Shady Deal at Sunny Acres" of the western Maverick, he is best known for his performance as Lars Hanson in forty episodes between 1974 and 1978 of NBC's Little House on the Prairie. He voiced the character of Merlin in Walt Disney's 1963 animated classic, The Sword in the Stone.
In 1967, Swenson played the role of U.S. President Theodore Roosevelt in the western film Brighty of the Grand Canyon, with co-stars Pat Conway and Joseph Cotten. Swenson appeared in a 1967 episode of Hogan's Heroes entitled "How to Win Friends and Influence Nazis", in which he played a likable and friendly German scientist, Dr. Karl Svenson, who is persuaded by Hogan (Bob Crane) to join the Allied war effort.
Swenson died of a heart attack in Torrington, Connecticut on October 8, 1978 shortly after filming the episode in which the Little House on the Prairie character Lars Hanson died. He was interred at Center Cemetery in New Milford, Connecticut.