Firth was born in Wirral, Merseyside, in the North West of England. He spent most of his time at his Warrington comprehensive school writing songs and it was only a couple of months before going to Cambridge to read English that he attended an Arvon Foundation course in West Yorkshire. This was run by Willy Russell and whilst on it, Firth had to write dialogue. He wrote about the only thing he knew - two sixteen-year-olds trying to write a song. Another course participant optioned it for his production company and Firth decided to become a writer. While at Cambridge he joined and composed songs for the Footlights where his contemporaries included David Baddiel who later invited him to contribute music to The Mary Whitehouse Experience on BBC radio. His first plays at this time were all directed by Sam Mendes.
On leaving Cambridge, he wrote and composed music for the Radio Four series And Now In Colour but was soon invited to meet Alan Ayckbourn in Scarborough and commissioned to write a play for the studio at for the Stephen Joseph Theatre. His one-act play A Man Of Letters was a success and led to the commissioning of a full-length play from Ayckbourn, Neville's Island, which later transferred to the West End and has been seen in translation all round the world in almost continuous production since its premiere.
During two successive Christmas runs, his stage musical version of his tv film The Flint Street Nativity at the Liverpool Playhouse became the most successful Christmas production in the theatre's history.
The play Calendar Girls, adapted by Firth from his own film, found favour with audiences across the UK during its 2008/09 tour, and in the process broke the all-time British box office record for a play and also continued to sell out during its West End residency. In 2010, the hit comedy embarked on a UK country wide tour, one of the largest ever for a play. Calendar Girls is also a success across the globe, playing in Germany, Finland, Norway, Denmark, Sweden, Israel, Greece, Argentina, France, Czech Republic, Belgium, Holland, Hungary, Poland, Spain, Australia, New Zealand, Canada, Russia, Bulgaria and Slovakia.. In 2011 it beat the all-time British box office record for a play, and has since raised over half a million pounds for Blood Cancer Charities, funding its own research project in Yorkshire hospitals. In an eighteen month window, Amateur Rights were snapped up by over 500 Amateur Dramatic groups across the UK and the rest of the world.
2013, marked the premiere of Firth's first solo musical, This Is My Family for which he wrote book, lyrics and music. Directed by Daniel Evans, it premiered at Sheffield Studio Theatre on 19 June 2013. This Is My Family won the UK Theatre Award for Best New Musical and Sian Philips won Best Supporting Actress.
In 2015, Tim Firth and Ivor Novello award winning songwriter Gary Barlow co-wrote The Girls, based on the story of the Calendar Girls. Directed by Roger Haines and Tim Firth, the new musical opened at Leeds Grand Theatre in November. The show then moved to The Lowry in Manchester in January 2016.
In all The Girls played 60 sell out performances, complete with 60 standing ovations, and was seen by just under 100,000 people. In January 2017, The Girls will open in London's West End, directed by Tim Firth.
In 2004 Firth released a solo album, Harmless Flirting, completing a tour of words and music with fellow playwright Willy Russell that won positive reviews at the Edinburgh Festival.
In March 2010, Firth was awarded an honorary Doctorate in Literature by the University of Chester.
Tim helps run the choir at Alvanley Primary School in Cheshire.