London was born John Griffith Chaney on January 12, 1876, in San Francisco, the illegitimate son of Flora Wellman, errant daughter of a well-to-do businessman, and "Professor" W. H. Chaney, a wandering astrologer. Jack's parents were not married at the time of his birth.
When he was eight months old, his mother married John London, a middle-aged widower with two daughters, Eliza and Ida. The boy became John Griffith London but was called Jack to distinguish him from his stepfather and used this nickname throughout life.
From his natural father Jack inherited a superb physique and literary gift. Although he had never known his father, like him, he developed a love for the sea and the belief that the ills of society could be cured by radical social reform.
From his mother, a neurotic woman whose mental balance had been affected by an illness in late youth, he acquired a temperament which caused him to alternate between elation and despair, the conviction that fair-haired Anglo-Saxons were the master race, and an irresponsibility about money. The period of Jack's youth was a time of nationwide economic depression and widespread unemployment.