He joined the Army in 1926 and was enlisted in the 60th King's Royal Rifle Corps, based at Tidworth Camp on Salisbury Plain, where he rose to the rank of Sergeant, becoming a physical training instructor.

[3] In 1931, his Army career came to an end, and after a short period playing with Barking Town in the Athenian League he joined Second Division Southampton in October 1931.

[3] He played two matches with Southampton's reserves, before being drafted into the first team as an emergency centre forward, where the "Saints" were suffering with an injury crisis with Bill Fraser, Arthur Haddleton and Johnny McIlwaine all unavailable.

Sillett scored twice on his debut, in a 3–1 victory at Burnley on 2 January 1932, and retained his place in the first team, playing up front until Ted Drake was promoted from the reserves in March.

[5] In 1933–34, he was again kept out of his favoured role as full back, this time by Arthur Roberts, but had a run of four games taking over from Tommy Brewis at inside right, during which he scored twice in a 5–0 victory over Bradford at The Dell on 16 December.

[10] In January, the Saints played Football League champions Sunderland in a Third Round FA Cup match in front of a record crowd of 30,380.

On 29 September 1942, Sillett was serving on HMS Registan with the rank of Able Seaman when it was torpedoed by German submarine U-332 140 miles east of Barbados, with the loss of 11 crew members and 5 DEMS gunners.