A former stand-up comedian,[1] he has written for a number of UK science fiction TV shows, namely Being Human, Dirk Gently and Doctor Who.

[2] He scripted the sci-fi comedy film Frequently Asked Questions About Time Travel, starring Chris O'Dowd, Dean Lennox Kelly, Marc Wootton, and Anna Faris.

[4] It is based on the popular comics anthology magazine Métal Hurlant, known in the United States as Heavy Metal and in Germany as Schwermetall.

Mathieson started his contributions to Doctor Who during the tenure of Peter Capaldi, working on series 8, 9 and 10.

In 2023, Mathieson independently published his first book, the short story collection The First Ten.