Jointly with Pemberton, created, wrote and starred in the sitcom Psychoville and the dark comedy anthology series Inside No.

He attended Andrew Marvell High School[1] and then Bretton Hall College of Education, where he met Mark Gatiss and Steve Pemberton, fellow actors and comedians.

[3] He appeared in two episodes of the award-winning pop-culture comedy Spaced as Robot Wars obsessed TA soldier Dexter and played neurotic Doctor Flynn in hospital sitcom TLC alongside Alexander Armstrong.

[4] In the 2008 English-language DVD release of the cult 2006 Norwegian animated film Free Jimmy, Shearsmith voiced the character of "Ante", a heavy-set, bizarrely dressed biker member of the "Lappish Mafia".

[5][6][7][8] The dark comedy series was written by Shearsmith and his League of Gentlemen writing partner Steve Pemberton.

[9][10] In 2011, Cameron Mackintosh's new musical Betty Blue Eyes opened in the West End, in which Shearsmith played downtrodden husband Gilbert Chilvers (a chiropodist) alongside Sarah Lancashire.

[11] In 2012, he appeared in Bad Sugar, a comedy pilot written by Sam Bain and Jesse Armstrong, along with Olivia Colman, Julia Davis and Sharon Horgan.

[14] In 2013, he played Patrick Troughton in An Adventure in Space and Time, a docu-drama about the conception and making of Doctor Who, which was written by Mark Gatiss.

[15] He worked with Vic Reeves and Bob Mortimer again in the first series of House of Fools as Martin the ghost and in the Christmas special as Santa.

[18][19] In 2015, he played Gagan Rassmussen in the Series 9 Doctor Who episode "Sleep No More", Steele in High-Rise directed by Ben Wheatley, Ray in Peter Kay's Car Share and Pastor John in the Christmas specials of Julia Davis' Hunderby.

[29] Shearsmith also appeared in the title role in The Dresser at the Chichester Festival Theatre, as well as reuniting with The League of Gentlemen for three television specials, transmitted on BBC2 in December 2017.

[30][31][32] Shearsmith appeared as himself in the 2018 short film To Trend on Twitter in aid of young people with cancer charity CLIC Sargent with fellow comedians David Baddiel, Steve Pemberton, Helen Lederer and actor Jason Flemyng.

[33] In 2020, he received a nomination for the Laurence Olivier Award for Best Actor in a Supporting Role for his performance as The President and Jon in A Very Expensive Poison at The Old Vic.