Mark St. John Ellis started his visual arts career as an art technician at the Douglas Hyde Gallery Trinity College Dublin installing such artists as Peter Doig and Felix Gonzalez Torres. From 1999 to 2009, he was the curator and co–ordinator of the Royal Hibernian Academy Ashford Gallery in Dublin and in the words of the Arts critic of the Irish Times, Aidan Dunne, one of the key figures in the contemporary art scene of Dublin responsible for the launching of many emerging artists.

From 2009 Mark St.John was the Programming Director and Layout Designer of International Arts Fairs for the Cross Gallery. In 2012 the basement space of that gallery became nagGallery founded by Mark St.John as the Creative Director. On the closure of the Dublin space in 2018 Mark St.John moved on to be the Curator and Creative Director of nagOffsite which went from being an Arts Consultancy facility in 2014 to an offsite curatorial programme of exhibitions in spaces such as Historic buildings. Mary Heffernan Head of OPW National Properties Ireland wrote in 2023 “Mark’s unique curatorial style and vision has delivered many of the most serene shows in Ireland over the past decades.” Mark St.John redesigned the State Collection of Modern and Contemporary Art in Farmleigh House and has on a continuing basis, curated, designed and photographed the State Collection of donated works from the International Academy of Ceramics in Rathfarnham Castle.