Bratby was born in Wimbledon, London in 1928, the first of two children.
His interest in art began when aged seventeen he was inspired by the teaching and encouragement his school art teacher Harold Watts, who recognized the potential in Bratby’s sketches. Bratby was conscripted but soon discharged due to his extreme myopia, but managed to obtain an ex-service grant, with which, at Watts’s encouragement, he went on to study at Kingston School of Art (1948–50), after which he studied at the Royal College of Art (1951–4), where he was awarded a bursary to travel to Italy. The professor of painting at the college, Carel Weight RA, thought him the most talented student he had ever had. On 2 April 1953 he married his fellow RCA student Jean Esme Oregon Cooke (1927–2008). A sculptor and potter, Cooke subsequently became a painter of real distinction was elected a Royal Academician in 1972.
Bratby worked in a harsh realist style, applying the paint thickly in vibrant colours, and portraying sometimes ugly and desperate faces in domestic settings. His vigorous realism was influenced by Van Gogh and Soutine This concern with social realism brought Bratby into contact with Jack Smith (1928-2011), Edward Middleditch (1923–87) and Derrick Greaves (b 1927), and these artists became the main exponents of the Kitchen Sink school. However, while the Kitchen Sink artists shared a desire to depict the banality of a working-class domestic environment, Bratby’s use of colours and his more middle-class surroundings distinguished his style from that of his peers.
His work was an immediate critical success. In 1954 aged 26 he had the first of a series of one-man exhibitions at the acclaimed Beaux Arts Gallery, London. In 1956 he was chosen to represent Britain at the Venice Biennale, along with the other kitchen sink painters: Edward Middleditch, Jack Smith, and Derrick Greaves. A major early painting, Still-Life with Chip Frier, was that year bought by the Tate Gallery. He won the John Moores junior section prize in 1957, and Guggenheim awards in 1956, 1957 and 1958. In 1954 he first exhibited a painting at the Royal Academy, becoming an associate in 1959, and a Royal Academician in 1971.
Bratby and Jean Cooke had four children together, all depicted in their parents’ artworks, and who were themselves artistically talented. Their marriage had broken down by the 1970s and they eventually divorced in 1977. In 1974 Bratby met Patricia (Patti) Prime (b. 1931), an actress. They married on 4 May 1977.
In the 1960s he embarked on an ambitious series of portraits which by the end of the 1980s numbered over 1500. In the 1970s he adopted a highly charged Fauvist palette. In the 1980s he painted cityscapes on trips abroad but concentrated on self-portraits and portraits of his second wife, in intimate poses and with bright colours and an economy of line.