Born in Livingston, West Lothian, Glen – a former pupil of Broxburn Academy – joined Heart of Midlothian from their youth academy system in 2006. Aged 16 he had a trial with Manchester United, but turned them down as he thought he was more likely to make it as a footballer with Hearts than with the Old Trafford club. Highly rated by the Hearts coaching staff, Glen earned his first involvement with the first team squad when he was an unused substitute in a league match at Motherwell on 5 March 2007. At the age of 16, he made his senior debut as a substitute against Dundee United on 17 March 2007.