Donnelly is a son of Raymond Donnelly, a sometime racing cyclist. Colin attended Eastwood High School, Newton Mearns and was a member of the Cambuslang Harriers. He showed some talent as a youngster, winning the Galloway and Renfrewshire Schools under-19s cross country championships. His first hill race was at Ben Lomond in 1978. The following year, he won the Ben Nevis Race and in 1980 finished a close second in the Three Peaks Race. He graduated in Arts from the University of Aberdeen.
The peak of Donnelly's running career was in the late 1980s. In 1986 he had another victory at Ben Nevis in one of the fastest times ever recorded for the race. He won the British Fell Running Championships three consecutive times from 1987 to 1989 and in 1988, he won the Snowdon Race.
Also in 1988, he set a record for the traverse of the Welsh 3000s with a time of 4:19 which stood until 2019 when it was beaten by Finlay Wild. Donnelly has also won the Welsh 1000 m Peaks Race many times.
He still holds the course records for the Buckden Pike Race, set in 1988, and the Shelf Moor Race, set in 1989. He continued to win races as late as 2017, thirty-eight years after his first Ben Nevis win.
He has completed the Bob Graham, Paddy Buckley and Ramsay Rounds, as well as the South Wales 2,000ers.
- Dave Woodhead, "Interview: Private and Personal", Up & Down, pilot issue (Jul 1990), 10-14.
- Thomson, Alan (28 September 1981). "A runner who can take mountains in his stride". The Glasgow Herald. p. 7. Retrieved 21 August 2016.
- Bill Smith, Stud Marks on the Summits (Preston, 1985), 395-96.
- Hugh Dan MacLennan, The Ben Race (Fort William, 1994), 166, 189-91.
- Steve Chilton, It's a Hill, Get Over It (Dingwall, 2013), 164-65.
- The Fell Runner, Sep 1988, 22.
- British Mountaineering Council: Finlay Wild Breaks Records for Welsh 3000ers and Snowdon Horseshoe.
- Muskett, Calum (27 November 2013). "Interview: Colin Donnelly, a Fell Running Great". www.ukclimbing.com. Retrieved 2 May 2018.
- WMRA: World Mountain Running Trophy 1989.
- WMRA: Masters World Mountain Running Championships, 2001.
- Barlick Fell Runners: Buckden Pike Fell Race Winners.
- Glossopdale Harriers: Shelf Moor Fell Race.
- Durham Fell Runners: Cronkley Fell Race 2017 Results.
- Calum Muskett on Vimeo: Colin Donnelly - Welsh 3000ers record (not filmed during the record run itself but on the route the following year).