Fagan started his professional career as a teenager at Celtic,[2] joining the club from Wellesley Juniors, the same route taken by his uncle John McFarlane 16 years earlier.
[3] He made his debut aged barely 18 in March 1935, and appeared five times for the Hoops as they won the 1935–36 Scottish Division One title; however, it was his six goals in as many matches at the start of the 1936–37 season which drew attention from other clubs.
While playing for the Reds, Fagan went straight into the line-up in his favoured inside-left position, although he did appear as a centre-forward; he ended his initial season at the club with nine goals in 36 matches.
He followed this up by scoring 15 times, one behind top scorer Berry Nieuwenhuys (who later invited Fagan to be best man at his wedding)[6] and missing only just three matches of the 1938–39 campaign.
He played as a wartime guest for Celtic,[4] Aldershot, Leicester, Northampton, Newcastle, Chelsea, Millwall and Reading.