John Harkins was serving his time in the Black Watch Regiment when he caught the eye of First Division Middlesbrough during an appearance for his regiment against Celtic in a Scottish Cup-tie. So impressed were the Teessiders that they bought Harkins out of the Army in September 1906.

He was quickly given his debut by Boro at centre-half, against Newcastle on October 1906. He missed just one game all season. Harkins played a further nine games the following season before being dropped after switching to right-half. He moved to non-league Broxburn Athletic in 1908 and on again to Bathgate in Scotland a year later.

In August 1910, he signed for Second Division Leeds City, with the Leeds Mercury describing him as 'A player of whom much is hoped'. He was immediately given his first-team debut and he was a regular that campaign, missing just seven games.

Harkins was just as consistent in 1911/12, but faded out of the picture following the arrival of Herbert Chapman as manager at the end of the season and he signed for non-league Darlington in 1912. Two years later he joined Coventry City, then managed by his former Leeds boss Frank Scott-Walford.

Harkins re-enlisted with the Black Watch after the onset of World War I and he fell in action at Amara in Iraq on 22 April 1916.