In 1893, he signed on to the sealing schooner Sophia 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 gruelling jobs in a jute mill and a street-railway power plant, he joined Kelly's industrial army and began his career as a tramp.

In 1894, he spent thirty days for vagrancy in the Erie County Penitentiary at Buffalo. In The Road, he wrote:

"Man-handling was merely one of the very minor unprintable horrors of the Erie County Pen. I say 'unprintable'; and in justice I must also say 'unthinkable'. They were unthinkable to me until I saw them, and I was no spring chicken in the ways of the world and the awful abysses of human degradation. It would take a deep plummet to reach bottom in the Erie County Pen, and I do but skim lightly and facetiously the surface of things as I there saw them."

A pivotal event was his discovery in 1895 of the Oakland Public Library and a sympathetic librarian, Ina Coolbrith (who later became California's first poet laureate and an important figure in the San Francisco literary community).

After many experiences as a hobo, sailor, and member of Kelly's Army he returned to Oakland and attended Oakland High School, where he contributed a number of articles to the high school's magazine, The Aegis. His first published work was "Typhoon off the coast of Japan", an account of his sailing experiences.

Jack London desperately wanted to attend the University of California and, in 1896 after a summer of intense cramming, did so; but financial circumstances forced him to leave in 1897 and he never graduated. Kingman says that "there is no record that Jack ever wrote for student publications" there.

In 1889, London began working from twelve to eighteen hours a day at Hickmott's Cannery. Seeking a way out of this gruelling labor, he borrowed money from his black foster mother Jennie Prentiss, bought the sloop Razzle-Dazzle from an oyster pirate named French Frank, and became an oyster pirate himself. In John Barleycorn he claims to have stolen French Frank's mistress Mamie. After a few months his sloop became damaged beyond repair. He switched to the side of the law and became a member of the California Fish Patrol.

While living at his rented villa on Lake Merritt in Oakland, London met poet George Sterling and in time they became best of 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 his aquiline nose and classical profile, and signed them as "Wolf". London was later to depict Sterling as Russ Brissenden in his autobiographical novel Martin Eden (1909) and as Mark Hall in The Valley of the Moon (1913).

In later life Jack London was a polymath with wide-ranging interests and a personal library of 15,000 volumes.

On July 25,1897,London and his brother in law James Shepard sailed to join the Klondike Gold Rush where he would later set his first successful stories. London's time in the Klondike, however, was quite detrimental to his health. Like so many others malnourished while involved in the Klondike Gold Rush, he developed scurvy. His gums became swollen, eventually leading to the loss of his four front teeth. A constant gnawing pain affected his abdomen and leg muscles, and his face was stricken with sores. Fortunately for him and others who were suffering with a variety of medical ills, a Father William Judge, "The Saint of Dawson", had a facility in Dawson which provided shelter, food and any available medicine. London's health recovered, but it was a unique twist of fate that London's life was perhaps saved by a Jesuit priest, since London was an agnostic.

London survived the hardships of the Klondike, Yukon Klondik, and these struggles inspired what is often called his best short story, "To Build a Fire". The famous version of this story was published in 1908; an early and radically different version was originally published in 1902. Labor, in an anthology, says that "To compare the two versions is itself an instructive lesson in what distinguished a great work of literary art from a good children's story. Labor (1994) 1902 version, famous 1908 version, The story concerned a Klondike prospector's stubborn futility in ignoring the dangers of nature, and in the end freezing to death when he is unable to build a simple fire that could save his life. London personally could probably closely identify himself with the man in the story, and must have seen this type of human folly many times in real life while in the Klondike.

His landlords in Dawson were two Yale and Stanford educated mining engineers Marshall and Louis Bond. Their father Judge Hiram Bond was a wealthy mining investor. The Bonds, especially Hiram, were active Republicans. Marshall Bond's diary mentions friendly sparring on political issues as a camp pastime.

Jack left Oakland a believer in the work ethic with a social conscience and socialist leanings and returned to become an active proponent of socialism. He also concluded that his only hope of escaping the work trap was to get an education and "sell his brains". Throughout his life he saw 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 Oakland in 1898, he began struggling seriously to break into print, a struggle memorably described in his novel, Martin Eden. His first published story was the fine and frequently anthologized "To the Man On Trail". When The Overland Monthly offered him only $5 for it—and was slow paying—Jack 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".

Jack London was fortunate in the timing of his writing career. He started just as new printing technologies enabled lower-cost production of magazines. This resulted in a boom in popular magazines aimed at a wide public, and a strong market for short fiction. In 1900, he made $2,500 in writing, the equivalent of about $75,000 today. His career was well under way.

Among the works he sold to magazines was a short story known as either "Batarde" or "Diable" in two editions of the same basic story. A cruel French Canadian brutalizes his dog. The dog out of revenge causes his death. London was criticized for depicting a dog as an embodiment of evil. He told some of his critics that man's actions are the main cause of the behavior of their animals and he would show this in another short story.

This short story for the Saturday Evening Post "The Call of the Wild" ran away in length. The story begins on an estate in Santa Clara and features a St. Bernard/Shepard mix named Buck. In fact the opening scene is a description of the Bond family farm and Buck is based on a dog he was lent in Dawson by his landlords. London visited Marshall Bond in California having run into him again at a political lecture in San Francisco in 1901.