Francis Sills was born in Baker Street, Marylebone, London on 14 September 1840. He was articled to Henry William Sass (1824-1874) in London. He then went as an improver to James George Smithers (1832?-1911) and to Sir Charles William Siemens (1823-1893).
In 1880 he was appointed executive engineer with the Bengal Department of Works in Calcutta, India and in 1882 married in St Paul's Cathedral, Calcutta. During his time in India, Sills carried out extensive building work in Calcutta, Dacca, and other parts of Bengal, and was engaged on alterations to the Calcutta Mint.
By 1892 he had returned to England and was living in Lee, Kent. From 1897 to 1905 he was in partnership with Percy Leeds (1871-?) in London. Work by the partnership included several business premises, warehouses, and dwellings, including St. Mildred's Vicarage in Lee.
Sills was elected an Associate of the Royal Institute of British Architects (ARIBA) in 1881. His address was given as 11 Amphill Place, Lambeth, Surrey in 1861 and 1871; Donington House, Norfolk Street, London in 1905; Norfolk Street, Strand London and Dornhurst, Sevenoaks, Kent in 1914; and Sonning, Calonne Road, Wimbledon, Surrey in 1919. He died in Kingston, Surrey, England on 15 September 1919.
Directory of British Architects 1834-1914. Compiled by Antonia Brodie, et al. Volume 2: L-Z. London; New York: British Architectural Library, Royal Institute of British Architects/Continuum, 2001