Donnelly's first acting was as Jason Winkler in Nickelodeon's tv series House of Anubis. He then guest starred in shows such as Doctors, Threesome, Misfits, Dancing on the Edge. In 2013, he landed a lead role in BBC's fantasy series Atlantis, where he played the role of Jason who washes up on the shores of the ancient and mysterious city of Atlantis. In 2014, he played the role of a cocky footballer Olly Hunter in the wholly improvised spoof documentary United We Fall. In 2016, he guest starred as Samuel Walker in the tv series Death in Paradise.