leeson began acting at the age of seven in the Independent Theatre Workshop.[5] His first roles were in films such as Batman Begins (2005),Shrooms (2007), and A Shine of Rainbows (2009).[6]In 2010, he appeared in a leading role in All Good Children. The reviewer for Variety magazineconsidered Gleeson "the pic's big discovery".[7]
Gleeson starred as Joffrey Baratheon in the first four seasons of the HBO series Game of Thrones.[8]He cites Joaquin Phoenix's portrayal of Commodusin Gladiator as an influence on his performance.[9]He is the founder and artistic director of Collapsing Horse Theatre Company, which is based in Dublin.
In 2012, Gleeson indicated an intention to retire from acting to pursue an academic career once his work on Game of Thrones was finished.[2]