 He served in the British Army during World War I.He was a director of the Eagle Star Insurance Company and Chairman of Vulcanite Ltd.

Sincliar served in the Royal Irish Rifles and Royal Irish Fusiliers during World War I, attaining the rank of Captain.In October 1937 he raised and commanded the Antrim Fortress Royal Engineers at Victoria Barracks, Belfast, one of the first Territorial Army (TA) units in Northern Ireland.