Some critics have assumed that the years Maugham spent studying medicine were a creative dead end, but Maugham felt otherwise. He was living in the great city of London, meeting people of a "low" sort whom he would never have met otherwise, and seeing them at a time of heightened anxiety and meaning in their lives. In maturity, he recalled the value of his experience as a medical student: "I saw how men died. I saw how they bore pain. I saw what hope looked like, fear and relief ..."
Maugham kept his own lodgings, took pleasure in furnishing them, filled many notebooks with literary ideas, and continued writing nightly while at the same time studying for his medical degree. In 1897, he published his first novel, Liza of Lambeth, a tale of working-class adultery and its consequences. It drew its details from Maugham's experiences as a medical student doing midwifery work in Lambeth, a South London slum. Maugham wrote near the opening of the novel: "... it is impossible always to give the exact unexpurgated words of Liza and the other personages of the story; the reader is therefore entreated with his thoughts to piece out the necessary imperfections of the dialogue."
Liza of Lambeth's first print run sold out in a matter of weeks. Maugham, who had qualified as a medic, dropped medicine and embarked on his 65-year career as a man of letters. He later said, "I took to it as a duck takes to water."
The writer's life allowed Maugham to travel and to live in places such as Spain and Capri for the next decade, but his next ten works never came close to rivalling the success of Liza. This changed in 1907 with the success of his play Lady Frederick. By the next year, he had four plays running simultaneously in London, and Punch published a cartoon of Shakespeare biting his fingernails nervously as he looked at the billboards.
Maugham's supernatural thriller, The Magician (1908), based its principal character on the well-known and somewhat disreputable Aleister Crowley. Crowley took some offence at the treatment of the protagonist, Oliver Haddo. He wrote a critique of the novel, charging Maugham with plagiarism, in a review published in Vanity Fair. Maugham survived the criticism without much damage to his reputation.
Popular success, 1914–39
By 1914, Maugham was famous, with 10 plays produced and 10 novels published. Too old to enlist when the First World War broke out, he served in France as a member of the British Red Cross's so-called "Literary Ambulance Drivers", a group of some 24 well-known writers, including the Americans John Dos Passos, E. E. Cummings, and Ernest Hemingway.
During this time, he met Frederick Gerald Haxton, a young San Franciscan, who became his companion and lover until Haxton's death in 1944. Throughout this period, Maugham continued to write. He proofread Of Human Bondage at a location near Dunkirk during a lull in his ambulance duties.
Of Human Bondage (1915) initially was criticized in both England and the United States; the New York World described the romantic obsession of the protagonist Philip Carey as "the sentimental servitude of a poor fool". The influential American novelist and critic Theodore Dreiser rescued the novel, referring to it as a work of genius and comparing it to a Beethoven symphony. His review gave the book a lift, and it has never been out of print since.
Maugham indicates in his foreword that he derived the title from a passage in Baruch Spinoza's Ethics:
- "The impotence of man to govern or restrain the emotions I call bondage, for a man who is under their control is not his own master ... so that he is often forced to follow the worse, although he see the better before him." 
Of Human Bondage is considered to have many autobiographical elements. Maugham gave Philip Carey a club foot (rather than his stammer); the vicar of Blackstable appears derived from the vicar of Whitstable; and Carey is a medic. Maugham insisted the book was more invention than fact. The close relationship between fictional and non-fictional became Maugham's trademark, despite the legal requirement to state that "the characters in [this or that publication] are entirely imaginary". In 1938 he wrote: "Fact and fiction are so intermingled in my work that now, looking back on it, I can hardly distinguish one from the other."