Rosoman was born in London and educated at the Deacon's school, Peterborough, and then at the King Edward VII school of art in Newcastle upon Tyne, under O' R. Dickey in 1930–4, at the R.A. Schools in 1935–6 and at the Central School under Bernard Meninsky in 1937–8. 
His first major break came in 1937, with a commission to illustrate My Friend Mr Leakey, a children's book by the scientist JBS Haldane. From 1938 he ran life classes at the Reinmann school, the London branch of a Berlin art college.
At the beginning of the Second World War he joined the Auxiliary Fire Service, and began making paintings based on his experiences.  One of these A House Collapsing on Two Firemen, Shoe Lane, London, EC4 (1940; now in the Imperial War Museum) shows an incident in which a young fireman who had just relieved Rosoman at at his position was killed.  The painting was shown in the "Firemen Artists" exhibition at the Royal Academy in 1941. He was seconded to the War Office to illustrate books on fire-fighting in 1943,  and was appointed an official war artist to the Admiralty the next year. Commissioned as a captain in the Royal Marines, he was posted to the Far East. He joined the aircraft carrier HMS Formidable in Sydney, and remained with the ship until the end of the war with Japan.
On his return to Britain he taught at Camberwell College of Art for a while before moving to Edinburgh College of Art in 1948 to teach mural painting. He organized a famous exhibition for Sergei Diaghilev at the Edinburgh festival of 1954, with the help of students, and made a large mural at the art college, where the exhibit was held (they later moved the exhibit to London.In 1956 he moved on to the Chelsea School of Art, and the following year to the Royal College of Art, where David Hockney was one of his students. 
In 1951 Rosoman painted a mural for the Festival of Britain on the South Bank in London and drew his first illustrations for the Radio Times. In 1958 he did the murals for the British Pavilion at the Brussels International Exhibition.
Rosoman was elected an associate of the Royal Academy in 1960 and became a full academician in 1969. He painted a mural at a restaurant in the Academy's home, Burlington House, depicting scenes of life within and around the building.
In 1988 he painted the ceiling of the chapel at Lambeth Palace.  His work there consists of a series of panels with scenes from the lives of St Augustine, Thomas Becket and Matthew Parker, and a Christ in Glory.
He received the OBE in 1981 . He was married twice.
Further reading 
- Michael Middleton, 'The Drawings of Leonard Rosoman', in Image; 3 (1949-1950), p.3-22
- "Leonard Rosoman RA (1913–2012) - Painters - Royal Academicians - Royal Academy of Arts". Royalacademy.org.uk. 1913-10-27. Retrieved 2012-03-04.
- Cathy Courtney. "Leonard Rosoman: Painter whose work profited from his oblique approach to life - Obituaries - News". The Independent. Retrieved 2012-03-04.
- Michael McNay. "Leonard Rosoman obituary | Art and design | guardian.co.uk". Guardian. Retrieved 2012-03-04.
- "Leonard Rosoman". Tate Gallery. Retrieved 5 October 2012.
- "A House Collapsing on Two Firemen, Shoe Lane, London, EC4". Imperial War Museum. Retrieved 6 October 2012.
- "Leonard Rosoman". The Telegraph. Retrieved 7 October 2012.
- "Inside Lambeth Palace". Archbishop of Canterbury. Retrieved 2012-10-04.