In 2000, Garth Marenghi's Fright Knight, a stage show written by Holness and Richard Ayoade, and starring Holness, Ayoade and Alice Lowe, was nominated for the Perrier Award at the Edinburgh Fringe. The show was built around Holness's spoof horror Writer character Garth Marenghi, with additional parts played by Ayoade and Lowe. The sequel to Fright Knight, Garth Marenghi's Netherhead, won the Perrier Award the following year.
From 2004, Holness played the regular character of Keith Bilk in the BBC Radio 4 series The Department, written by Chris Addison (series 1, 2), John Oliver, and Andy Zaltzman.
Holness's comedy character, Merriman Weir, a guitar-picking folk singer, has performed at small comedy clubs in London and around the UK. Merriman Weir has also appeared as a guest in the pilot episode of the Channel 4 comedy chat show Man to Man with Dean Learner (2005) (then known as Deano's After Dark) where he was interviewed by Garth Marenghi's publicist Dean Learner—a character created by Richard Ayoade. Holness is an exceptionally accomplished Guitarist and this shows in the episodes featuring Weir.
In 2006 he appeared in the BBC Two comedy programme Time Trumpet. In 2009 he appeared in the Channel 4 sitcom Free Agents.
In 2010 he appeared in the Ricky Gervais and Stephen Merchant film Cemetery Junction. In 2011 he played a minor role in the first episode of Friday Night Dinner as 'Chris Parker'.
In 2011 he appeared in Life's Too Short, a sitcom written by Ricky Gervais and Stephen Merchant and starring Warwick Davis.
In 2012 he wrote and directed a one-off play for Sky Arts Playhouse Presents series entitled The Snipist. It starred Douglas Henshall in the titular role, protecting the citizens of a dystopic alternative 1970s Britain against the threat of rabies. John Hurt provided 'The Voice of The Ministry'.
In 2017 he appeared in Simon Blackwell's 'Back', during flashbacks portraying the recently deceased father of the main character.
