 He made 192 appearances for the Tynecastle Park club in the Scottish Football League and Scottish Cup prior to the outbreak of World War I.As a reservist, Sinclair had pledged to join up and was one of the first to leave Hearts to go off to the war in August 1914 (later the whole squad enlisted – the first British club to do so; most of the players were with "C" Company of the Royal Scots – Sir George McCrae's Battalion).Sinclair served with the Royal Field Artillery, saw action at the Battle of Mons and was injured in 1915, but survived the war and returned to play for Hearts again, along with Paddy Crossan and Bob Mercer.