[3][4] A traditional outside right (winger), Sinclair had been a soldier in his teenage years before joining Leith in 1906; he signed for Hearts at the end of the 1907–08 season.
[5] 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,[6] saw action at the Battle of Mons and was wounded and discharged in 1915.
He won three Scotland caps[8] prior to WWI and also played for the Scottish Football League XI twice, either side of the conflict.
This biographical article related to association football in Scotland, about a forward born in the 1880s, is a stub.