John Randall Bratby was born in Wimbledon. He did not show an aptitude for art until he was seventeen when his teacher Harold Watts recognised his potential in several sketches of boxers he had completed. Encouraged by Watts, he enrolled at Kingston School of Art in 1947. He remained there until 1950 and, a year later, started at the Royal College of Art, where he studied until 1954. Whilst studying there, he lived an unsettled, often wild lifestyle, sometimes begging and sleeping rough in Hyde Park. His work regularly reflected this lifestyle. Inspired by the realism of Sickert, he painted the seedier aspects of his life and surroundings in a correspondingly crude manner.

In 1953, Bratby married fellow student Jean Esme Oregon Cooke, who later went on to become a successful painter in her own right. In the following year, he had the first in a series of solo exhibitions at the Beaux Arts Gallery and he exhibited for the first time at the Royal Academy. Beginning to gain public recognition, he won several prizes such as two Guggenheim awards and  he was chosen to represent Britain at the Venice Biennale in 1956. He became an associate of the Royal Academy in 1959, eventually becoming a Royal Academician in 1971.

Bratby became disillusioned with painting throughout most of the 1960s and abandoned art in order to concentrate on writing. However, by 1967 he was painting again, when he began a series of portraits which eventually numbered over 1500 works. Nearing the end of his life, he tended to work frenetically although his paintings rarely lived up to the mastery of his earlier works. His work is represented in the collections of the Tate and the Victoria and Albert Museum in London, and in the Museum of Modern Art in New York.