Ainsworth initially studied law but left it for literature, publishing his first novel anonymously in 1826. His first success came with the novel Rookwood (1834), featuring the highwayman Dick Turpin, which led many reviewers to hail him as the successor to Sir Walter Scott. Jack Sheppard (1839), the story of an 18th-century burglar, was equally successful, but it helped to stir up fierce reaction against the “Newgate” school of novel writing—of which Ainsworth and Edward Bulwer-Lytton (Lytton, Edward George Earle Bulwer-Lytton, 1st Baron) were considered exemplars—for its supposed glamorization of crime. Thereafter Ainsworth switched to historical novels based on places rather than criminals, including The Tower of London (1840), Old St. Paul's, a Tale of the Plague and the Fire (1841), Windsor Castle: An Historical Romance (1843), and The Lancashire Witches (1849). In a long career that extended to 1881, he published some 40 novels.

Ainsworth was editor of Bentley's Miscellany from 1839 to 1841, and he owned that periodical from 1854 to 1868. He was also editor of The New Monthly Magazine (1845–70) and his own Ainsworth's Magazine (1842–54). His novels made him a wealthy man, but his ventures as an editor and publisher were generally unsuccessful. His novels excel in conveying the pageantry and bustle of history but lack coherence of plot and subtlety of characterization. Between 1836 and 1845 Ainsworth's novels were illustrated, with great distinction, by George Cruikshank (Cruikshank, George).