Having served from 1908 to 1913 in the Territorial Army (Army No. 1554) with 54th Div. RE, Sydney Brown enlisted in the Royal Engineers in November 1914. Between 1914 and 1918 he served with various units (Army No. 524621, including East Anglian Field Coy RE, and in May 1918 he joined 69 Field Coy RE. He was appointed L/Cpl on 8 August 1918. On 29 March 1919 he was transferred to Class "Z", Army Reserve, on demobilisation.

On 30 October 1924 he re-enlisted into the RAMC (Army No. 7354126) as a Private, and in November 1924 was appointed into the RAMC Supplementary Reserve. In October 1933 he was discharged (as a Private) on termination of his engagement, after 9 years' service.

Then, in December 1939 he enlisted into the Territorial Army at Bedford, aged 49, and was appointed a Clerk in the Royal Engineers, and posted to the Depot Batallion at Chatham. He was finally released to the Royal Army Reserve in August 1948, aged almost 58.