He has performed at the Edinburgh Festival Fringe, both as a solo act and as part of the comedy group Cowards, and plays Alan Partridge's sidekick Simon in film and television.

[4][5] There he met Tom Basden, Stefan Golaszewski, and Lloyd Woolf, with whom he formed the sketch group Cowards.

Key co-starred in Daniel Kitson's play Tree when it premiered in September 2013 at the Royal Exchange, Manchester.

[9] He appeared alongside Paul Ritter and Rufus Sewell in Yasmina Reza's Art at The Old Vic, directed by Matthew Warchus, from December 2016 to February 2017.

Like The Slutcracker, it featured Key reading "deliberately bad" poetry interspersed with black-and-white films.

[11] In 2023, Key toured his sell-out show Mulberry around the UK and Ireland,[12] as well as a run at the SoHo Playhouse in New York City.

Key's prior radio projects included Cowards and Mark Watson Makes the World Substantially Better.

In 2010, Key was heard as Duncan in the radio sitcom Party, created by Tom Basden and based on the stage show of the same name.

[16] Key first appeared on television in a comic role in 2006's satirical comedy Time Trumpet, as an Eastenders special effects supervisor.

He also appeared in an episode of Saxondale alongside future Mid Morning Matters with Alan Partridge co-star Steve Coogan.

[17] As part of the show's bonus online content, the BBC uploaded videos of Key and Watson playing No More Women, a parlour game they had invented several years earlier, with Horne supplying narration.

That year he also appeared as Ray, nemesis and old archery teammate of Paul (Jim Howick) in an episode of friend Tom Basden's BBC1 sitcom, Here We Go.

[22] Key reprised his role as Sidekick Simon for Steve Coogan's Alan Partridge film Alpha Papa, released in August 2013.

In 2020, Key collaborated with designer Emily Juniper to create He Used Thought As a Wife (An Anthology of Poems and Conversations from Inside).