Dennis Morgan was born Earl Stanley Morner in the village of Prentice in Price County in northern Wisconsin, the son of Grace J. (née Vandusen) and Frank Edward Morner. He was of Swedish descent on his father's side.
After relocating to Los Angeles, California, Morgan starred in the war-time dramas, God Is My Co-Pilot and Captains of the Clouds, opposite Ginger Rogers in Kitty Foyle and Perfect Strangers, the comedy Christmas in Connecticut, with Barbara Stanwyck and Sydney Greenstreet, and the musical adventure, The Desert Song.
Morgan was a top leading man at Warner Brothers in the 1940s, starring with best friend Jack Carson in many movies, several of which were "two guys" buddy pictures. His peak years were 1943 to 1949.
He appeared in sporadic television guest roles in the 1950s, including the ABC religion anthology series, Crossroads, in the 1955 episode "The Gambler". He was cast as Dennis O'Finn in the 1958 episode "Bull in a China Shop" on Alfred Hitchcock Presents. In 1959, Morgan appeared as a regular, Dennis Chase, in eleven episodes of the crime drama, 21 Beacon Street.
By 1956, he had retired from films but still made occasional appearances on television, such as the role of Chad Hamilton in the 1962 episode "Source of Information" of the short-lived NBC newspaper drama series, Saints and Sinners. In 1963, he portrayed Dr. Clay Maitland in "The Old Man and the City" on NBC's The Dick Powell Theater. In 1968, was cast as Dennis Roberts in the episode "Bye, Bye, Doctor" of the CBS sitcom, Petticoat Junction. His final screen performance was on March 1, 1980, as Steve Brian in the episode "Another Time, Another Place/Doctor Who/Gopher's Engagement" of ABC's The Love Boat. Jane Wyman and Audrey Meadows appeared in the same episode.
In 1958, Morgan spearheaded the drive to establish a new public park in La Crescenta in Los Angeles County. He dedicated Two Strike Park on July 4, 1959, named for Morgan's belief that "a kid with no place to play already has two strikes against him"