Horne was born on 6 September 1978 in Nottingham, the son of Brian Horne and Glenis Reynolds. He grew up in Burton Joyce, Nottinghamshire. Horne has an elder brother, Daniel. He attended Burton Joyce Primary School. He studied A level performing arts at Southwell Minster School in Nottinghamshire, and has a degree in drama from the University of Manchester. Here, Horne met Bruce Mackinnon in the first few weeks of term; they realised that they shared a similar sense of humour, but did not write material together until the third year of their studies.

Horne began his career as a comedian and later became one half of the comedy duo Mat and MacKinnon – first performing at the Edinburgh Festival in 2000. He was spotted at the festival by Catherine Tate who invited him to perform in The Catherine Tate Show. Since then Horne has continued to appear in several Catherine Tates Nan specials. He reprised his role as Jamie in two festive episodes of Catherine Tates Nan in 2015.

In 2003, he starred in Channel 4's 20 Things to do Before You're 30 and a year later earned the role of Ben, an atheist RE teacher, in the Channel 4 series Teachers.

He also appeared in the first series of ITV production Doc Martin. In 2005, Horne appeared in an episode of The Smoking Room, playing Dominic the nephew of Janet who was conducting a psychological study of the employees.

In 2007 he starred in the BBC sitcom Roman's Empire and in November that year appeared in comedy sketches in The Kylie Show on ITV1 alongside Kylie Minogue, playing her assistant. Horne was also a guest star in the second series of Robin Hood.

From 2007 to 2010 he starred in the double BAFTA winning series Gavin & Stacey as main character Gavin Shipman, a kind-hearted young man who meets up with a girl he meets through their work phone calls and falls in love with her. The programme ran for three series with a Christmas special and received widespread critical acclaim.

In 2008, Horne appeared in a modern adaptation of the Three Billy Goats Gruff as part of the BBC's Fairy Tales season. He also narrated the ITV2 series The Passions of Girls Aloud.

Horne starred in the film Lesbian Vampire Killers alongside James Corden and made his stage debut in a 2009 revival of Entertaining Mr Sloane at the Trafalgar Studios. On 3 April 2009 it was reported that Horne collapsed on-stage during a West End performance of the play. He remained on-stage until medical assistance arrived, at which point he was taken to hospital. A statement released soon after said that he was exhausted after working 'flat out' for 3 months and it was later revealed that he was suffering from a virus.

Horne also guest-starred in the video for the Maccabees' 2009 free-download single "No Kind Words"

On 18 February 2009, he presented the 2009 BRIT Awards alongside James Corden and Kylie Minogue.

In 2010, he appeared in an episode of Miss Marple, named "The Secret of Chimneys", as Bill Eversleigh. Horne appeared alongside fellow Gavin & Stacey star Ruth Jones.

He played the Culture Club drummer, Jon Moss, in the 2010 BBC docudrama about the teenage years of Boy George, Worried About the Boy.^[19]

In 2016 he toured British theatres with Catherine Tate in The Catherine Tate Show Live.^{[citation needed]}

Appeared as young Mr. Grace in "Are You Being Served?" 2016.

Since working on the BBC sitcom Gavin & Stacey, he has collaborated a number of times with its co-creator James Corden^[20]

Horne and Corden presented the Big Brother spin-off show Big Brother's Big Mouth^[21] as guests in 2007 and were then regulars for the run covering E4's Big Brother: Celebrity Hijack

Together they presented the NME Shockwaves Awards^[22] in 2008 and a section of Comic Relief in March 2009.

In 2008 Horne and Corden toured the UK with a stand-up show in preparation for their BBC sketch show Horne & Corden.^[23] The first episode was broadcast on show of 10 March 2009 on BBC Three. It was presented in front of a live audience, in a style reminiscent of Morecambe and Wise or The Two Ronnies. A variety of pre-filmed sketches and live performances were included.

2009 also saw the release of Lesbian Vampire Killers, a comedy horror film. Reviews of the film were largely negative.^[24] It holds a 29% approval rating on review aggregator Rotten Tomatoes with an average score of 4/10.^[25] James Christopher of The Times described it as "profoundly awful", stating that it is an "instantly forgettable lads' mag farce" and an "appalling waste of a perfectly decent title".^[26] Allan Hunter of the Daily Express called it "badly written and hastily executed" and "takes all the easy options of bad taste, bosoms and body fluids".^[27] Anthony Quin writing in The Independent gave the film 1 star out of 5, describing it as woeful and stating that Horne and Corden had "overstretched their appeal" and looked in danger of becoming today's Hale and Pace.^[28] Peter Bradshaw of The Guardian described the film as "mostly pretty awful, but there are one or two crass laughs".^[29]

The Sun, however, gave it a positive review; Sun reviewer the Sneak gave Lesbian Vampire Killers a rating of 3 out