Stelios Messinesos de Syllas [commonly known as Leo de Syllas] was born at Northbrook, Holmwood, Surrey, England on 24 July 1917. In 1933 he entered the Bartlett School of Architecture, University College, London, but transferred to the Architectural Association School in London in 1936. During his time at the AA he was editor of the school's student-led magazine Focus (1938–39).
In 1939 de Syllas, with ten fellow graduates of the Architectural Association formed the Architects’ Co-operative Partnership (ACP) [from 1951 known as the Architects' Co-Partnership (ACP)], an architectural co-operative. Within months the ACP was dissolved following the outbreak of World War Two. de Syllas then joined the research and experiments department of the Ministry of Home Security.
In 1943 he went to the British West Indies to work as an assistant to the architect Robert Joesph Gardner-Medwin (1907-1995) in the Colonial Development and Welfare Organisation to work on a programme of buildings for education, housing, and health, and in 1946 was appointed architect and planning officer to the government of Barbados. The following year he designed a training college in the grounds of Erdiston House in Bridgetown, Barbados. In 1947 de Syllas returned to Britain and rejoined the newly re-formed ACP. Work by the ACP was seldom attributed to individuals, however it is known that he worked on the design of a number of schools including Leesbrook School Secondary Modern, at Chaddesden in Derbyshire, completed in 1955.
In 1954 de Syllas with fellow ACP partner Michael Grice (1917-2008)) opened an ACP office in Lagos, Nigeria where over the next eight years they designed schools, office blocks. They also designed a large housing development in Akosombo, Ghana. The Nigerian office closed in 1962. In 1963 de Syllas designed a rooftop extension to Simpson's department store in Piccadilly, London. He was also largely responsible for the design of the St. Paul's Cathedral Choir School in London, completed in 1967.
On 30 September 1964, while working on a project for new schools for the Tunisian government de Syllas was killed in an automobile accident near Le Kef, Tunisia.
‘Architects’ Co-Partnership’. Journal of the Royal Institute of British Architects vol. 74, 1967 pp. 229-238
Cox, Anthony. Architects Co-Partnership: the first 50 years. Potters Bar:, Hertfordshire Architects Co-Partnership, 1989
Powers, Alan. ‘Chapter 8. Architects’ Co-Partnersip’ in in British Design: Tradition and Modernity after 1948, edited by Ghislaine Wood. London: Bloomsbury Academic, pp. 113-126