Theodore Gregg was born in Islington, Middlesex. England in May 1876. He was articled to his father, Ebenezer Gregg (1832?-1901) from 1892 to 1895 and remained as his assistant until 1897. He commenced independent practice as an architect in London in 1899. He was in partnership with his father from 1899 to 1901 and with Lionel Gordon Detmar (1880-1910) as Gregg & Detmar from 1905 to 1910.
Gregg was elected a Licentiate of the Royal Institute of British Architects (LRIBA) in 1911. On 23 July 1914, he left London for Melbourne, Victoria, Australia. It is not known if he returned to England. His name does not appear in the Kalendar of The Royal Institute of British Architects after this date.
38 Parkway, Gidea Park, London Borough of Havering in 1911. With Detmar: Maida Vale American Skating Palace [later BBC Maida Vale Studios,] Delaware Road, London in 1909-10.
Directory of British Architects 1834-1914. Compiled by Antonia Brodie, et al. Volume 1: A-K. London; New York: British Architectural Library, Royal Institute of British Architects/Continuum, 2001
'Maida Vale Rink Opened'. The Era, 26 February 1910, p. 27