Harold Conybeare Trimnell was born in Old Malden, Surrey, England on 19 April 1874. He was articled to Sidney Robert James Smith (1858-1913) from 1893 to 1896 and remained with him as his assistant. He also attended classes at the Architectural Association in London and was awarded the AA silver medals in 1892 and 1895; and King's College, London. He qualified as an architect in 1897 and was elected an Associate of the Royal Institute of British Architects (ARIBA) later that year.
Trimnell was active as an architect from c.1897, first in London and then, from c.1900, in Woldingham, Caterham Valley, Surrey. In c.1905-06 he was in partnership with William Rupert Davison (1881-1947) as Trimnell & Davison. They were awarded second place in a competition to design the Central Library in Hackney, London. Designs for their entry are illustrated in British Competitions in Architecture vol. 1, no. 6, January 1906 (pp. 9-11). Trimnell died in West Moors, Dorset on 5 May 1958.
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 [Note: it is stated that Trimnell was in partnership with T. Raffles Davison. This appears to be incorrect. He was in partnership with William Rupert Davison (1881-1947)]