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, as the curator and co–ordinator of the Royal Hibernian Academy Ashford Gallery in Dublin he was responsible for the reputation of the Ashford Gallery in its support of emerging artists. The Arts Editor of the Irish Times, Aidan Dunne, wrote that Mark St.John, through his work with the Ashford, has become one of the key figures in the contemporary art scene of Dublin and is responsible for the launching of many emerging artists, and in December 2017 wrote that he was “a curator of immense experience and flair. “
Key exhibitions that he curated in the Ashford Gallery were ‘all colours black ( next to nothing )’ and 'nothing without a woman’ in 2007, 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.
When Mark St.John teamed up with the Cross Gallery in 2009, he curated ' Angels of Perversity’ and 'Airports for Shadows' with 'New Forms of Beauty' in response to Dublin Contemporary in 2011. He founded nagGallery in 2012.