According to Flora Wellman's account, as recorded in the San Francisco Chronicle of June 4, 1875, Chaney demanded that she have an abortion. When she refused, he disclaimed responsibility for the child. In desperation, she shot herself. She was not seriously wounded, but she was temporarily deranged. After giving birth, Flora turned the baby over for care to Virginia Prentiss, an African-American woman and former slave. She was a major maternal figure throughout London's life. Late in 1876, Flora Wellman married John London, a partially disabled Civil War veteran, and brought her baby John, later known as Jack, to live with the newly married couple. The family moved around the San Francisco Bay Area before settling in Oakland, where London completed public grade school.

In 1885, London found and read Ouida's long Victorian novel Signa. He credited this as the seed of his literary success. In 1886, he went to the Oakland Public Library and found a sympathetic librarian, Ina Coolbrith, who encouraged his learning. (She later became California's first poet laureate and an important figure in the San Francisco literary community).

In 1889, London began working 12 to 18 hours a day at Hickmott's Cannery. Seeking a way out, he borrowed money from his foster mother Virginia Prentiss, bought the sloop Razzle-Dazzle from an oyster pirate named French Frank, and became an oyster pirate himself. In his memoir, John Barleycorn, he claims also to have stolen French Frank's mistress Mamie. After a few months, his sloop became damaged beyond repair. London hired on as a member of the California Fish Patrol.

In 1893, he signed on to the sealing schooner Sophie Sutherland, bound for the coast of Japan. When he returned, the country was in the grip of the panic of '93 and Oakland was swept by labor unrest. After grueling jobs in a jute mill and a street-railway power plant, London joined Coxey's Army and began his career as a tramp. In 1894, he spent 30 days for vagrancy in the Erie County Penitentiary at Buffalo, New York. In The Road, he wrote:

London joined the Socialist Labor Party in April 1896. In the same year, the San Francisco Chronicle published a story about the twenty-year-old London giving nightly speeches in Oakland's City Hall Park, an activity he was arrested for a year later. In 1901, he left the Socialist Labor Party and joined the new Socialist Party of America. He ran unsuccessfully as the high-profile Socialist candidate for mayor of Oakland in 1901 (receiving 245 votes) and 1905 (improving to 981 votes), toured the country lecturing on socialism in 1906, and published two collections of essays about socialism: The War of the Classes (1905) and Revolution, and other Essays (1906).

On July 12, 1897, London (age 21) and his sister's husband Captain Shepard sailed to join the Klondike Gold Rush. This was the setting for some of his first successful stories. London's time in the harsh Klondike, however, was detrimental to his health. Like so many other men who were malnourished in the goldfields, London developed scurvy. His gums became swollen, leading to the loss of his four front teeth. A constant gnawing pain affected his hip and leg muscles, and his face was stricken with marks that always reminded him of the struggles he faced in the Klondike. Father william Judge, "The Saint of Dawson", had a facility in Dawson that provided shelter, food and any available Medicine to London and others. His struggles there inspired London's short story, "To Build a Fire" (1902, revised in 1908), which many critics assess as his best.

London left Oakland with a social conscience and socialist leanings; he returned to become an Activist for socialism. He concluded that his only hope of escaping the work "trap" was to get an education and "sell his brains". He saw his writing as a Business, his ticket out of poverty, and, he hoped, a means of beating the wealthy at their own game. On returning to California in 1898, London began working to get published, a struggle described in his novel, Martin Eden (serialized in 1908, published in 1909). His first published story since high school was "To the Man On Trail", which has frequently been collected in anthologies. When The Overland Monthly offered him only five dollars for it—and was slow paying—London came close to abandoning his writing career. In his words, "literally and literarily I was saved" when The Black Cat accepted his story "A Thousand Deaths", and paid him $40—the "first money I ever received for a story".

London married Elizabeth "Bessie" Maddern on April 7, 1900, the same day The Son of the Wolf was published. Bess had been part of his circle of friends for a number of years. She was related to stage actresses Minnie Maddern Fiske and Emily Stevens. Stasz says, "Both acknowledged publicly that they were not marrying out of love, but from friendship and a belief that they would produce sturdy children." Kingman says, "they were comfortable together... Jack had made it clear to Bessie that he did not love her, but that he liked her enough to make a successful marriage."

In a letter dated Dec 27, 1901, London's Macmillan publisher George Platt Brett, Sr. said "he believed Jack's fiction represented 'the very best kind of work' done in America."

While living at his rented villa on Lake Merritt in Oakland, CA., London met poet George Sterling; in time they became best friends. In 1902, Sterling helped London find a home closer to his own in nearby Piedmont. In his letters London addressed Sterling as "Greek", owing to Sterling's aquiline nose and classical profile, and he signed them as "Wolf". London was later to depict Sterling as Russ Brissenden in his autobiographical novel Martin Eden (1910) and as Mark Hall in The Valley of the Moon (1913).

Despite being frequently attributed to London, the passage does not appear at all in the extensive collection of his writings at Sonoma State University's website. However, in his book The War of the Classes he published a 1903 speech entitled "The Scab", which gave a much more balanced view of the topic:

London shared Common concerns among European Americans in California about Asian immigration, described as "the yellow peril"; he used the latter term as the title of a 1904 essay. This theme was also the subject of a story he wrote in 1910 called "The Unparalleled Invasion". Presented as an historical essay set in the Future, the story narrates events between 1976 and 1987, in which China, with an ever-increasing population, is taking over and colonizing its neighbors with the intention of taking over the entire Earth. The western nations respond with biological warfare and bombard China with dozens of the most infectious diseases. On his fears about China, he admits, "it must be taken into consideration that the above postulate is itself a product of Western race-egotism, urged by our belief in our own righteousness and fostered by a faith in ourselves which may be as erroneous as are most fond race fancies."

In 1905, London purchased a 1,000 acres (4.0 km) ranch in Glen Ellen, Sonoma County, California, on the eastern slope of Sonoma Mountain, for $26,450. He wrote: "Next to my wife, the ranch is the dearest thing in the world to me." He desperately wanted the ranch to become a successful Business enterprise. Writing, always a commercial enterprise with London, now became even more a means to an end: "I write for no other purpose than to add to the beauty that now belongs to me. I write a book for no other reason than to add three or four hundred acres to my magnificent estate."

In 1906, the New York World published "deadly parallel" columns showing eighteen passages from London's short story "Love of Life" side by side with similar passages from a nonfiction article by Augustus Biddle and J. K Macdonald, titled "Lost in the Land of the Midnight Sun". London noted the World did not accuse him of "plagiarism", but only of "identity of time and situation", to which he defiantly "pled guilty".

The couple also visited Goldfield, Nevada, in 1907, where they were guests of the Bond brothers, London's Dawson City landlords. The Bond brothers were working in Nevada as mining Engineers.

The most serious charge of plagiarism was based on London's "The Bishop's Vision", Chapter 7 of his novel The Iron Heel (1908). The chapter is nearly identical to an ironic essay that Frank Harris published in 1901, titled "The Bishop of London and Public Morality". Harris was incensed and suggested he should receive 1/60th of the royalties from The Iron Heel, the disputed material constituting about that fraction of the whole novel. London insisted he had clipped a reprint of the article, which had appeared in an American newspaper, and believed it to be a genuine speech delivered by the Bishop of London.

An amateur boxer and avid boxing fan, London reported on the 1910 Johnson-Jeffries fight, in which the black boxer Jack Johnson vanquished Jim Jeffries, known as the "Great White Hope". In 1908, London had reported on an earlier fight of Johnson's, contrasting the black boxer's coolness and intellectual style, with the apelike appearance and fighting style of his Canadian opponent, Tommy Burns:

Stasz notes that "London regarded the Wobblies as a welcome addition to the Socialist cause, although he never joined them in going so far as to recommend sabotage." Stasz mentions a personal meeting between London and Big Bill Haywood in 1912.

In 1913 and 1914, a number of newspapers printed the first three sentences with varying terms used instead of "scab", such as "knocker", "stool pigeon" or "scandal monger".

Beginning in December 1914, London worked on The Acorn Planter, A California Forest Play, to be performed as one of the annual Grove Plays, but it was never selected. It was described as too difficult to set to music. London published The Acorn Planter in 1916.

The Iron Heel is interesting as an Example of a dystopian novel that anticipates and influenced George Orwell's Nineteen Eighty-Four. London's socialist politics are explicitly on display here. The Iron Heel meets the contemporary definition of soft science fiction. The Star Rover (1915) is also science fiction.

The biographer Stasz notes that the passage "has many marks of London's style" but the only line that could be safely attributed to London was the first. The words Shepard quoted were from a story in the San Francisco Bulletin, December 2, 1916 by Journalist Ernest J. Hopkins, who visited the ranch just weeks before London's death. Stasz notes "Even more so than today journalists' quotes were unreliable or even sheer inventions" and says no direct source in London's writings has been found. However, at least one line, according to Stasz, is authentic, being referenced by London, and written in his own hand, in the autograph book of Australian suffragette Vida Goldstein:

London witnessed animal cruelty in the training of circus animals, and his subsequent novels Jerry of the Islands and Michael, Brother of Jerry included a foreword entreating the public to become more informed about this practice. In 1918, the Massachusetts Society for the Prevention of Cruelty to Animals and the American Humane Education Society teamed up to create the Jack London Club, which sought to inform the public about cruelty to circus animals and encourage them to protest this establishment. Support from Club members led to a temporary cessation of trained animal acts at Ringling-Barnum and Bailey in 1925.

London's literary executor, Irving Shepard, quoted a Jack London Credo in an introduction to a 1956 collection of London stories:

This passage as given above was the subject of a 1974 Supreme Court case, Letter Carriers v. Austin, in which Justice Thurgood Marshall referred to it as "a well-known piece of trade union literature, generally attributed to author Jack London". A union newsletter had published a "list of scabs," which was granted to be factual and therefore not libelous, but then went on to quote the passage as the "definition of a scab". The case turned on the question of whether the "definition" was defamatory. The court ruled that "Jack London's... 'definition of a scab' is merely rhetorical hyperbole, a lusty and imaginative expression of the contempt felt by union members towards those who refuse to join", and as such was not libelous and was protected under the First Amendment.

In 1996, after the City of Whitehorse, Yukon, renamed a street in honor of London, protests over London's alleged racism forced the city to change the name of "Jack London Boulevard" back to "Two-mile Hill".

Some nineteen original collections of short stories were published during London's brief life or shortly after his death. There have been several posthumous anthologies drawn from this pool of stories. Many of these stories were located in the Klondike and the Pacific. A collection of Jack London's San Francisco Stories was published in October 2010 by Sydney Samizdat Press.

Ambrose Bierce said of The Sea-Wolf that "the great thing—and it is among the greatest of things—is that tremendous creation, Wolf Larsen ... the hewing out and setting up of such a figure is enough for a man to do in one lifetime." However, he noted, "The love element, with its absurd suppressions, and impossible proprieties, is awful."