Acaster attended Montagu Secondary School in Kettering and went on to study music at Northampton College.
Acaster previously played drums in local bands in Northamptonshire including Pindrop, Three Line Whip and The Wow! Scenario. When Acaster was a member of the Capri Sun Quartet he was known as William Strawberry. After The Wow! Scenario broke up, he "decided to do comedy before deciding what he really wanted to do".
In 2009 Acaster performed in a show at the Edinburgh Festival Fringe with fellow comedians Josh Widdicombe and Nick Helm.
In 2010 he supported Josie Long on tour.
In 2011 he supported Milton Jones on tour.
He appeared on Russell Howard's Good News Extra and was the internet act in Dave's One Night Stand from Bristol. In 2011 he had his first solo show at the Edinburgh Festival, "Amongst Other Things". Acaster appeared in Australia Versus and was co-host in My First Gig on Resonance FM radio. He appeared in Chris Addison's E4 programme Show and Tell in autumn 2011.
In spring 2012 he performed "Amongst Other Things" as solo shows in various venues in the UK.
Acaster's 2012 Edinburgh show was called "Prompt". It began its run to positive reviews and enthusiastic audience response. It received a nomination from the Foster's Edinburgh Comedy Awards for best comedy show at the festival in 2012.
Acaster performed "Prompt" at 14 venues in England and Wales between January and March 2013, after which he appeared at Auckland Comedy Festival in New Zealand. He won the New Zealand Comedy Guild's best international act award in December 2013.
Acaster's 2013 Edinburgh show "Lawnmower" was staged at the Pleasance Courtyard. It was nominated for the Foster's Edinburgh Comedy Award 2013 for best comedy show. In September 2013 he made his first appearance on the BBC musical comedy panel show Never Mind the Buzzcocks. He completed his UK tour of "Lawnmower" at a hometown gig in Kettering on 30 November 2013.
Acaster starred in various segments of radio, including the half-hour programme "James Acaster's Findings - Bread" for BBC Radio 4, also featuring Nathaniel Metcalfe and Bryony Hannah. A series of four episodes of "James Acaster's Findings" was recorded in June and July 2014, the first of these episodes was broadcast on 5 November 2014. Acaster also regularly appeared on Josh Widdicombe's XFM radio show, where he has a keen following for his "classic scrapes".
Acaster's 2014 Edinburgh show was called Recognise. The show was previewed in April and May in Australia and New Zealand, where it won the New Zealand International Comedy Festival Award for Best International Show. Recognise was again nominated for the Foster's Edinburgh Comedy Award 2014 for best comedy show - third in a row for Acaster. It was performed at over 30 British venues during autumn 2014, including a two-week run at the Soho Theatre in December due to the success of the October dates at the same venue.
Acaster appeared as a panellist on 8 Out of 10 Cats on 17 March and 4 November 2014, and on Spicks and Specks in Australia on 14 May 2014. Acaster has been a guest several times on Mock the Week.
Acaster's 2015 Edinburgh show was called Represent, again performed at the Pleasance Courtyard. It earned Acaster his 4th consecutive Edinburgh Comedy Award nomination for Best Show, becoming the second artist to do so after Al Murray. The UK tour of this show ran from October to December 2015, ending at the Lyric Theatre, Shaftesbury Avenue. Due to the show's success, Acaster took "Represent" to the Soho Theatre for a week-long run in March 2016.
Acaster appeared as a guest on The Dog Ate My Homework in March 2015 and Have I Got News for You in June 2015. On 2 December 2015, he appeared as "Mike" on the BBC Three sitcom Josh. On 7 December 2015, he appeared on Live at the Apollo, performing segments from his show "Represent".
During this year he won the Chortle awards for Best Breakthrough Act and Best Show for Recognise.
On 11 February 2016, Acaster appeared as a guest on Room 101, where he attempted to have geese and the shot put sent to Room 101. On 12 February 2016, he appeared as a guest on QI. On 16 February 2016, he was a panelist on The Dog Ate My Homework.
Acaster's 2016 show was called "Reset". It was previewed at the Melbourne International Comedy Festival, and the New Zealand International Comedy Festival in Auckland and Wellington, both in April/May 2016, then at the Udderbelly Festival in June 2016 and finally at The Tringe Festival in Tring in July. Acaster performed this show at the Edinburgh Festival Fringe in August 2016, where he became the first comedian to receive a fifth nomination for best show at the festival. He toured the Reset show in autumn 2016, mostly at venues he appeared at in 2015 but in 2016 the venues were sold out.
Acaster appeared on the BBC Three online special Live from the BBC on 2 March 2016.
In June 2016, Acaster appeared as a guest on Russell Howard's Stand Up Central. In July 2016, Acaster performed in the comedy tent at Latitude Festival.
In September 2016 he wrote a pilot episode of a new sitcom "We The Jury" which was broadcast by the BBC.
In 2017 Acaster is taking "Recognise", "Represent" and "Reset" to various venues, performing one each night over three nights as a "Trelogy". A further show of previously performed material, "Recap", is being developed to help tie the shows together thematically. He is expected to have a book "Classic Scrapes" published in August this year, followed by a book-reading tour in autumn 2017 .