In 1883, Horsley was a founder member of the St. George's Art Society, 1884 of the Art-Workers' Guild. He was the first recipient, in 1887 and again in 1888, of the Owen Jones Studentship of the Royal Institute of British Architects (RIBA) which took him twice to Italy.
A RIBA associate in 1890, he left the institute in 1892 in protest against plans for compulsory registration of architects; he rejoined as a fellow in 1906.
From 1911 to 1913, Horsley was president of the Architectural Association.
Horsley was first influenced by the Arts and Crafts Movement and built a number of country residences. His best known buildings are in a Baroque revival style following the examples of Christopher Wren. In this style he designed St Paul's Girls' School in Hammersmith (1901–1902; Music School added in 1914) and a few stations for the North Western Railway such as Hatch End station (1911).
Horsley was also an excellent draughtsman and submitted views of historic monuments to the annual exhibitions of the Royal Academy all his life. He also prepared presentation drawings for architect colleagues. Horsley was the illustrator of A History of Gothic Art in England by E. S. Prior (published 1900).
- A.S. Gray, Edwardian architecture. A biographical dictionary, London, Duckworth, 1985.
- Alastair Service, Edwardian architecture. A handbook to building design in Britain 1890–1914, London, Thames and Hudson, 1977 (The world of art library)
- Dictionary of Scottish Architects 1840–1980