Leonard Manasseh studied at the Architectural Association and in 1941 was elected an Associate of the Royal Society of British Architects. During the war he serve as a pilot in the Fleet Air Arm.
From 1946 to 1948 he was assistant architect at Hertfordshire County Council Architects Department and following that he became a senior architect for the Stevenage Development Corporation. In 1950 he set up his own practice following the Festival of Britain Award and during the 1950's taught at the Architectural Association. From 1960 to 1965 he became President of the Architectural Association and has served as council member on numerous architectural organisations. In 1982 he was awarded an OBE and in 1989 he was elected President of the RWA.
Leonard Manasseh's partnership has won several awards for architecture including the RIBA Award, Southern Region in 1973, the Structural Steel Design Award in 1974 and the same year the National Heritage "Museum of the Year Award". In 1975 they received the European Architectural Heritage Year Award and in 1975 the RICS and The Times Conservation Award (2nd) for Wellington Country Park.
During his prominent career he has also published numerous articles in architectural magazines and together with Roger Cunliffe published 'Office Buildings' for B.T. Batsford Ltd.