McCahill began his senior career at Boghead playing for Dumbarton and spent nearly five years there, making nearly 150 league appearances for the club McCahill impressed in a Scottish Cup tie against Celtic in January 1989 and signed for the club a couple of days later. However, McCahill sustained a fractured jaw in league match against Motherwell on 12 April 1989.
McCahill featured rarely for Celtic after that and left on a free transfer to Morton in October 1992.
McCahill made his debut for Celtic on 25 February 1989, coming on as a second-half substitute in a 3-0 win over Dundee at Dens Park. He made a further 4 league appearances for Celtic that season and also played in the Dubai Champions Cup match against Liverpool on 4 April 1989, in which he turned in a highly impressive performance which helped Celtic to a win on penalties after the match had finished 1-1. McCahill played regularly for Morton for the next few years and won a Scottish Second Division league medal in 1995.
