Widdicombe was born in Hammersmith, London and grew up in Haytor Vale near Widecombe-in-the-Moor in Devon. He then attended South Dartmoor Community College and later studied linguistics at the University of Manchester.
Before becoming a comedian, Widdicombe worked as a sports journalist, writing for The Guardian.
Widdicombe's elder half-brother, Henry, is a comedy promoter and one of the founders of the Machynlleth Comedy Festival.
Widdicombe began performing live in 2008, and made it to the final of "So You Think You're Funny" at the Edinburgh Fringe Festival during the same year. In 2009 he co-wrote and performed in the sketch show Superclump, and appeared with James Acaster and Nick Helm.
In 2010, Widdicombe worked as a contributor for the Dora the Explorer magazine, and in 2011 he performed his debut solo show "If This Show Saves One Life..." at the Edinburgh Fringe Festival, and was subsequently nominated for Best Newcomer by the Foster's Edinburgh Comedy Awards and for the Malcolm Hardee "Act Most Likely to Win a Million Quid" Award.
In 2012, Widdicombe became a regular on Stand Up for the Week and in July he made his debut appearance on Mock the Week. He was also a main contributor to Channel 4's daily alternative review of each day's events at the London 2012 Summer Paralympics, The Last Leg with Adam Hills, alongside Adam Hills and Alex Brooker. He also appeared on The Last Leg of the Year, the end of year special of the show. In September 2012, Widdicombe appeared on The Million Pound Drop with The Last Leg co-host Brooker. The pair won £100,000 for their selected charities. His chosen charity was Scope. He has pectus excavatum, which he discovered after mentioning it to Christian Jessen on an episode of 8 Out of 10 Cats, is asthmatic and suffers from severe hayfever.
In 2013, Widdicombe was again a regular on The Last Leg, after Channel 4 renewed the show for nine further episodes, beginning on 25 January 2013. A third series started on 31 July 2013, a fourth on 31 January 2014 and a fifth on 1 August 2014. In May 2013 Widdicombe was a guest on the second series of Dara Ó Briain: School of Hard Sums with Marcus Brigstocke. Following this, he featured as a comedian on The Apprentice spin-off show, The Apprentice: You're Fired! In June 2013 he again appeared as a panellist on Mock the Week, featuring on several episodes of the show's 12th series. In 2014 Widdicombe appeared again on Mock the Week and on QI and also made his debut on Have I Got News for You. Widdicombe won Celebrity Mastermind broadcast 31 December 2013 with a score of 24, on the specialist subject of his favourite band, Blur. His chosen charity was The Lily Foundation.
On 1 July 2014, the BBC announced Widdicombe would be joining its Radio 5 Live team from August 2014 as the new host of Fighting Talk along with Georgie Thompson. Both hosts announced on 10 August 2016 that they will not be returning to the show for series 14 in September due to their "increasingly busy schedules."
In August 2014, Widdicombe starred in the BBC Three "Comedy Feeds" pilot Josh, which he co-wrote with comedian Tom Craine and which was directed by David Schneider. A full six-episode season of Josh, also directed by Schneider, ran on BBC Three from 11 November through 16 December 2015. BBC Three ordered a second full season of Josh; the new series was filmed during April to June 2016 and will air on BBC Three sometime in the autumn.
In 2015, Widdicombe starred as a regular contestant on the Dave game show Taskmaster and won the first series. For one of the tasks, Widdicombe got a tattoo of host Greg Davies's name on his left foot. He then returned for a team task in series two where he partnered Richard Osman and Jon Richardson.
Widdicombe has performed on radio on BBC Radio 4's Arthur Smith's Balham Bash and Absolute Radio's The Frank Skinner Show, written for comedy panel shows Mock the Week, 8 Out of 10 Cats and BBC Radio 4's Look Away Now, and supported stand-up comedians Russell Howard, Michael McIntyre, Alan Carr, Stephen Merchant and Shappi Khorsandi on their respective live tours.
Widdicombe hosted a weekly radio show on XFM (now Radio X) with Neil "Producer Neil" Fearn from February 2013 to July 2015, which was broadcast initially on Saturdays but which moved to Sundays in August 2014 coinciding with his new role as Fighting Talk presenter (see above). The show regularly featured contributions from comedians James Acaster, Nish Kumar, Ivo Graham and several others along with occasional interjections and background laughter by XFM's "Intern Charles". The podcast of the show was named "iTunes Best New Audio Podcast" in 2013. The last edition of the show aired on 26 July 2015.
Widdicombe has made four appearances on Live at the Apollo. He was a featured performer on Episodes 7.8 (January 21, 2012), 9.1 (November 22, 2013), and 11.7 (AKA Nöel at the Apollo; December 20, 2015) and hosted on Episode 12.2 (November 14, 2016).
In December 2015, Widdicombe announced that he was a team captain on the BBC Two comedy panel show Insert Name Here, appearing alongside Richard Osman and host Sue Perkins. The show aired in January 2016 and returned for a second series in January 2017. Josh claims to have seen Ocean Colour Scene live 28 times in the past seven years.Live: And Another Thing (18 November 2013)
What Do I Do Now... Live (28 November 2016)