Robert Mitchum was born in Bridgeport, Connecticut in 1917 into a Norwegian-Irish Methodist family. His mother Ann Harriet Gunderson was a Norwegian immigrant and sea captain's daughter; his father James Thomas Mitchum was a shipyard and railroad worker of Irish descent. His older sister, Annette (known as Julie Mitchum during her acting career), was born in 1914. Their father James Mitchum was crushed to death in a railyard accident in Charleston, South Carolina in February 1919, when Robert was less than two years old and Annette was not yet five.
Acting
Mitchum arrived in Long Beach, California in 1936, staying again with his sister Annette, now going by the name of Julie. She had migrated to the West Coast in the hope of acting in movies. Soon, the rest of the Mitchum family joined them in Long Beach. During this time, Mitchum worked as a ghostwriter for astrologer Carroll Righter. His sister Julie convinced him to join the local theater guild with her. In his years with the Players Guild of Long Beach, Mitchum made a living as a stagehand and occasional bit-player in company productions. He also wrote several short pieces which were performed by the guild. According to Lee Server's biography (Robert Mitchum: Baby, I Don't Care), Mitchum put his talent for poetry to work writing song lyrics and monologues for Julie's nightclub performances.  In 1940, he returned to Delaware to marry Dorothy Spence, and they returned to California. He remained a footloose character until the birth of their first child James, nicknamed Josh. They had two more children: Chris and Petrine. Mitchum got a steady job as a machine operator with the Lockheed Aircraft Corporation.  Mitchum suffered a nervous breakdown (which resulted in temporary blindness), apparently from job-related stress. He sought work as an actor or extra in films. His agent got him an interview with Harry Sherman, the producer of Paramount's Hopalong Cassidy western film series which starred William Boyd; Mitchum was hired to play minor villainous roles in several films in the series during 1942 and 1943. In 1943 he and Randolph Scott were soldiers in the Pacific Island war film Gung Ho!  Mitchum continued to find work as an extra and supporting actor in numerous productions for various studios. After impressing director Mervyn LeRoy during the making of Thirty Seconds Over Tokyo, Mitchum signed a seven-year contract with RKO Radio Pictures. He was groomed for B-Western stardom in a series of Zane Grey adaptations.  Following the moderately successful Western Nevada, Mitchum was lent from RKO to United Artists for The Story of G.I. Joe (1945). In the film, he portrayed war-weary officer Bill Walker (based on Captain Henry T. Waskow), who remains resolute despite the troubles he faces. The film, which followed the life of an ordinary soldier through the eyes of journalist Ernie Pyle (played by Burgess Meredith), became an instant critical and commercial success. Shortly after making the film, Mitchum was drafted into the United States Army, serving at Fort MacArthur, California. At the 1946 Academy Awards, The Story of G.I. Joe was nominated for four Oscars, including Mitchum's only nomination for Best Supporting Actor. He finished the year with a Western (West of the Pecos) and a story of returning Marine veterans (Till the End of Time), before filming in a genre that came to define Mitchum's career and screen persona: film noir.
When did he start acting?
1942 and 1943.
1942 and 1943.