Mark St. John Ellis started his visual arts career as an art technician at the Douglas Hyde Gallery in Dublin installing such artists as Peter Doig and Felix Gonzalez Torres. From 2000 to 2009, he was the curator and co–ordinator of the Royal Hibernian Academy Ashford Gallery in Dublin and responsible for the reputation of the Ashford Gallery in its support of emerging artists. The Arts critic of the Irish Times, Aidan Dunne, wrote that Mark St.John, through his work with the Ashford Gallery, was one of the key figures in the contemporary art scene of Dublin and was responsible for the launching of many emerging artists. In December 2017 he wrote that he was “a curator of immense experience and flair. “ Key exhibitions that he curated in the Ashford Gallery were in 2007 ‘all colours black ( next to nothing )’ and 'nothing without a woman’, and the annual emerging graduates exhibitions selected from the degree shows. He was the co-ordinator of the AIB Emerging Artist Award from its creation to 2009.

In 2009 Mark St.John became Programme Director to the Cross Gallery and curated many exhibitions including 'Angels of Perversity’. Airports for Shadows' and 'New Forms of Beauty' were in response to Dublin Contemporary in 2011.

In 2012 he was the founding Creative Director of nagGallery and on the closure of the Dublin based space in 2018 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 non white cube gallery spaces such as Historic buildings. Currently nagOffsite is in the research and development stages of building a gallery space with accommodation in rural County Cavan.