Fagan took over as Brisbane coach in 2017, when the Lions were second bottom on the ladder. They won five games in each of his first two seasons, then reached the finals for the first time in 2019 and have not missed September since.
Saturday's grand final was the club's fourth straight, and the Lions completed a three-peat with a remarkable comeback against Fremantle. Fagan departs having coached the team into the past four grand finals for three premierships.
A Tasmanian, Fagan did not play at the top level, but was a premiership player with Hobart and Devonport. He began his coaching career in 1998 when the late Neale Daniher appointed him Melbourne's reserves coach. He later became an assistant coach at Hawthorn and then the club's football boss, playing a key role alongside senior coach Alastair Clarkson in the Hawks' three-peat between 2013 and 2015.
His retirement marks the end of a remarkable rise from a club at its lowest ebb to one of the great modern AFL dynasties.