TheatreNI was established following a merger between the Ulster Association of Youth Drama (UAYD) and the Northern Ireland Theatre Association (NITA).UAYD was the lead body in Northern Ireland which supported and promoted youth theatre and drama for young people aged 11-25, while the NITA was the representative body for professional theatre in Northern Ireland. NITA was constituted in April 2004, following its previous incarnation as the Theatre Producers Group (NI).

In Spring 2015 these two organisations merged to form TheatreNI as both recognised the alignment between their respective services and programmes to individuals and organisations working within the professional theatre and youth drama sectors. Both UAYD and NITA established strong relationships with their respective members and gained sound reputations for delivering accessible and high-quality development opportunities and providing effective representation to policy makers and funders.