MacIntosh was born in Merionethshire. He was educated at Repton School in Derbyshire, and studied linguistics at the University of Edinburgh. While at university, he was actively involved in the Edinburgh University Theatre Company and The Improverts.
MacIntosh's big break in television when he was cast as the deadpan accountant Keith Bishop in The Office. Besides The Office, MacIntosh has also appeared on various shows such as Miranda and Little Britain. He formed a double act with Tim FitzHigham and they performed at the Edinburgh Fringe Festival in 2007. In 2009, MacIntosh appeared alongside several other celebrities in a charity music video for the Chris Rea song "Driving Home for Christmas". In early 2013, he prepared to play at a new West End show Geek, A New Musical at the Tristan Bates Theatre, which opened on 22 April 2013. He is also a regular on the radio sitcom Cabin Pressure, as Carl, the bored ATC operator. In 2013 he appeared in a national TV advert for ao.com. A year later he appeared as Barry the window cleaner in the film The Confusion of Tongues (2014).
MacIntosh has signed on with Salopian Films for the lead part in a new comedy six-part series called For the Love of Ella. The series also stars Lucy Drive, Alex Reid, Bobby Ball, Daniel Peacock, Melanie Sykes, Darren Day, Francoise Pascal and Billy Pearce. The script was written by Simon W. Golding (co-producer) and directed by James Farina (co-producer).
- BBC - Comedy: The Office - Character Profile - Keith
- "Packing a punch for Red Nose Day". Burton Mail. 24 February 2005.
- "Absolute Bedlam". Edit, page 13. Winter 2010.
- Edinburgh Festival Guide – All Aboard
- "Finding Fatimah official website". Retrieved 3 Feb 2017.
- IMDb - https://www.imdb.com/title/tt3293568/combined