He impressed the show's expert judges (who included Nigel Lythgoe and Nicki Chapman) enough to win selection as a founder member of the pop group Hear'Say, alongside Danny Foster, Myleene Klass, Kym Marsh and Suzanne Shaw.
After the split of Hear'Say, Sullivan appeared in several musical theatre productions including Fame, Love Shack and What a Feeling and performed at the Royal Gala opening of the Wales Millennium Centre.
In 2003, Sullivan appeared in Grease as Danny Zuko opposite Claire Buckfield as Sandy and Amanda-Jane Manning as Rizzo at the Jersey Opera House.
Sullivan sang with Jessica Cervi and Sarah O'Connor in the final of RTÉ One's Fame: The Musical in Dublin on 13 June 2010.
[3] Sullivan took over from Alex Gaumond in the lead role of Galileo in We Will Rock You in the West End Production of the musical at the Dominion Theatre.
[citation needed] Beginning in May 2013, Sullivan shared the role of Tick/Mitzi with Jason Donovan in the UK tour of Priscilla, Queen of the Desert.