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. Their mother was awarded a government pension; she soon realized she was pregnant; her and James' second son John was born in September of that year.
Ann married again, to Major Hugh Cunningham Morris, a former Royal Naval Reserve officer. He helped care for her three children. Ann and Morris also had a daughter together, Carol Morris, born July 1927 on the family farm in Delaware. When all of the children were old enough to attend school, Ann found employment as a linotype operator for the Bridgeport Post.
As a child Mitchum was known as a prankster, often involved in fistfights and mischief. When he was 12, his mother sent him to live with her parents in Felton, Delaware.