Born in Dundee, Scotland to Liz and Peter McColgan, both athletes, McColgan is 1.8 metres (5 ft 11 in) tall and weighs 53 kilograms (117 lb). She is a member of Dundee's Hawkhill Harriers Club and is coached by her mother, former 10,000 metres World Champion and Olympic silver medallist Liz McColgan.
In 2006 she won a silver medal at the Scottish School's Championships in the 1500 metres and a bronze medal at the International Under-17s Schools Championships. In 2007 she came fifth in the Scottish Senior Championships 1500 metres. At the 2008 Scottish Indoor Championships she won a bronze medal in the 1500 metres.
McColgan was selected for the 2008 Commonwealth Youth Games in India but suffered a major knee injury from which took her a year-and-a-half to recover. She also missed the 2011 World Championships, held in Daegu, South Korea, after breaking her foot at a Diamond League event in London. Her senior debut for Great Britain came at the 2011 European Team Championships in Stockholm, Sweden. Initially selected as a travelling reserve to cover a range of events, she was called upon to compete in the 3000 metres steeplechase, an event which she had only competed in twice before, and ran a personal best while finishing ninth. She came fourth in the 1500 metres at the 2011 UK Indoor Championships and won the silver medal in the 5000 metres at the UK Outdoor Championships. She won the gold medal in the 1500 metres at the 2011 Scottish University Championships.
She won two gold medals at the 2012 Scottish Universities Indoor Championships, placing first in both the 1500 and 3000 metres events. In June 2012 at a meeting in Oslo, Norway she ran a time of nine minutes 38.45 seconds to beat the Olympic 'A' qualification standard in the 3000 metres steeplechase. McColgan confirmed her qualification for the steeplechase at the 2012 Summer Olympics in London by winning the British trials in a time of nine minutes 56.90 seconds. She was the only athlete to finish in under ten minutes and the only British athlete to have achieved the 'A' qualifying standard. After achieving Olympic qualification McColgan chose not to compete at the 2012 European Athletics Championships in Helsinki, Finland. She competed for Great Britain team for the 2012 Summer Olympics in the women's 3000 metres steeplechase alongside compatriot Barbara Parker. Her time of 9:54.36 was not sufficient for her to reach the final.
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- Graham Fraser (2 September 2012). "Eilish McColgan – Life after the Olympics and the Great Scottish Run". STV. Retrieved 3 September 2012.