Jonjo O'Neill is an actor from Northern Ireland known for his stage and television work. O'Neill was born in Belfast, grew up in the Whiterock Road area and was educated at St Mary's Grammar School. Growing up he was passionate about musical theatre and was a member of Ulster Youth Theatre and performed with the Ulster Theatre Company. In 1996 at the age of 18 he won a place and a full scholarship to the Guildford School of Acting, and moved to England. His first television role was the year he graduated from drama school, in Extremely Dangerous (1999).

A member of the Royal Shakespeare Company (RSC) 2009-2011 ensemble, his roles included Mercutio in Romeo and Juliet, Orlando in As You Like It, and Launcelot in Morte D'Arthur. His performances during the RSC's six-week residency at Park Avenue Armory in New York were hailed as "forceful" and "irresistible." At the 2012 World Shakespeare Festival in Stratford-upon-Avon, O'Neill played the title role in Roxana Silbert's production of Richard III at the Swan Theatre.

In 2012 he won praise for his performance in Lucy Prebble's play The Effect at the Royal National Theatre headlining alongside Billie Piper, whom he later appeared alongside in the 2013 fiftieth anniversary episode of Doctor Who: "The Day of the Doctor". He also appeared in "The Mortal Remains," the final vignette in the Coen brothers's film The Ballad of Buster Scruggs (2018).