Gleeson began acting at the age of 8, in the Independent Theatre Workshop. His first roles were in films such as Reign of Fire (2002), Batman Begins (2005), Shrooms (2007), and A Shine of Rainbows (2009). In 2010, he appeared in a leading role in All Good Children. The reviewer for Variety magazine considered Gleeson ‘the pic’s big discovery’.

Gleeson attended Trinity College Dublin between 2010 and 2015. He studied philosophy and theology and was elected a scholar at the university in 2012. At Trinity, Gleeson was a member of DU Players, where he met his future co-founders of Collapsing Horse Theatre Company.

In 2012, Gleeson indicated an intention to retire from acting to pursue an academic career once his work on Game of Thrones was finished. In 2014, Gleeson retired from acting after concluding his work in Game of Thrones. In an interview, he stated that while he had previously been interested in pursuing academia, he had since ‘gone off that idea.’

Gleeson is a founder, producer and company member of Collapsing Horse Theatre Company, based in Dublin. He was part of the original cast in the company’s first theatre production Monster / Clock, a children’s theater show which premiered in Dublin in 2012. He next appeared in the company’s ‘lo-fi comedy’ Bears in Space which premiered in Dublin in July 2014 and took part of the 2014 Edinburgh Festival Fringe. The show was well reviewed and had revival productions in Dublin and London in 2015 and off-Broadway in New York in September 2016 as part of Origin’s 1st Irish theatre festival.

In May 2018, Gleeson publicly supported the repeal of Ireland’s Eighth Amendment.

In 2019, Gleeson made two public appearances. In June, he was featured in the musical-comedy program AMUSICAL at the Kilkenny Cat Laughs comedy festival in Kilkenny, Ireland with comedians Eleanor Tiernan and Alison Spittle. In August, he made a surprise appearance at Over the Top Wrestling’s Trinity Brawl 2 event in Dublin where he played the face, aptly called ‘TV’s Jack Gleeson,’ opposite Irish wrestler J Money.

In March 2020, it was announced Gleeson would return to acting, having been cast in Sara Pascoe’s new series Out Of Her Mind.