His brother, Norman, played senior football for West Perth and the state team, and the two other brothers, Robert and Ron, played junior and reserves football for West Perth.

He played his first match for Western Australia against in the 1924 Australasian Football Carnival in Hobart.

McDiarmid was made vice-captain of Western Australia for the 1930 carnival, and was again named vice-captain for the 1933 Carnival,[3] held in Sydney, but missed a large portion of the 1933 season due to a knee injury, and was unable to take part.

[5] In total, McDiarmid played 183 games of senior WAFL football, 177 for West Perth and six for Claremont, as well as representing his state on 25 occasions.

[6] McDiarmid was one of the largest WAFL players at the time, in terms of both height and size, standing just under 6 feet (1.8 m), and weighing 15 stone 10 pounds (100 kg) at his peak.