Leech was educated at Charterhouse, where he met William Makepeace Thackeray, who was to be his lifelong friend. He then began to study medicine but soon drifted into the artistic profession and in 1835 published Etchings and Sketchings by A. Pen, Esq., comic character studies from the London streets. In 1840 Leech began contributing to magazines with a series of etchings in Bentley’s Miscellany; he also collaborated with George Cruikshank, whose work his own resembled in both style and subject. Later, however, he excluded the horrific and satirical elements present in the tradition of English caricature established in the late 18th and early 19th centuries by James Gillray and Thomas Rowlandson. Leech developed in his caricatures a comfortable, warmly humorous middle-class urbanity, in which character is underlined by emphatic contrasts of stock types. These qualities emerge from the four etchings illustrating Charles Dickens’ Christmas Carol (1844), the Comic History of England (1847–48), and the woodcuts for the Comic History of Rome (1852). These were followed by numerous etchings and woodcuts of sporting scenes in the novels of his friend R.S. Surtees.
Leech’s first contribution to Punch appeared in the issue of Aug. 7, 1841. This was the beginning of a fruitful connection that resulted in about 3,000 caricatures and other illustrations for the magazine. Leech concentrated on social caricature, as in Pictures of Life and Character from the Collection of Mr. Punch (1854, 1860, and 1863). Leech and the English illustrator Sir John Tenniel were the creators of the conventional image of John Bull—a jovial and honest Englishman, solid and foursquare, sometimes in a Union Jack waistcoat and with a bulldog at heel. He also contributed to Punch almanacs and pocketbooks, to Once a Week, and to The Illustrated London News, as well as to numerous novels and miscellaneous volumes.