[1] He stated that he was joining UKIP out of a desire to see "fundamental change in British politics" and because he believed "many of those at the top of the Conservative Party are simply not on our side.

"[2] In 2016, Carswell said that he had "jumped ship with the express goal of changing the image of UKIP and ensuring that it was an asset rather than a liability in the referendum campaign ... to decontaminate the brand".

[citation needed] His father, Wilson Carswell, a Scottish doctor and Fellow of the Royal College of Surgeons, diagnosed the first confirmed Ugandan cases of HIV/AIDS in the early 1980s,[10] and was one of a number of people engaged in drawing attention to the unfolding pandemic.

[8] His father's experiences in Uganda were among the inspirations for the character Dr Nicholas Garrigan in Giles Foden's novel The Last King of Scotland.

[13][14] Carswell was educated at two independent boarding schools for boys: St Andrews School in Turi in Kenya in East Africa, and Charterhouse in Godalming, Surrey, in southern England,[15][16] followed by the University of East Anglia (UEA), where he was taught by Edward Acton, and graduated with an upper second-class honours bachelor's degree in history in 1993.

[16] At the 2001 general election, Carswell contested Sedgefield, the constituency of Prime Minister Tony Blair, as the Conservative candidate.

In the months before the 2005 general election, Carswell worked in the Conservative Party's Policy Unit,[16] then run by David Cameron.

In 2008, he took part in an Armed Forces Parliamentary Scheme trip to Afghanistan, after which he called for more resources to be allocated to British troops serving there.

[21] In December 2009, Carswell tabled a Bill in the House of Commons calling for a public referendum on Britain's membership of the European Union (EU).

[24] In February 2010, he asked Gus O'Donnell to suspend Cabinet meetings held outside London,[25] when it was found that the government was using them to host Labour Party events in marginal seats.

[50] After being elected in 2005, Carswell originally designated a £1 million flat in London as his second home, and claimed over £21,000 for food, rent and furniture.

[53] Conservative Party commentator and Daily Telegraph columnist Charles Moore credits Carswell, together with MEP Daniel Hannan, as the architects behind the idea of a Great Repeal Bill, as well as the concept of a "Contract with Britain" offered during the election, the "recall" of MPs who have displeased their constituents, open primaries for the selection of parliamentary candidates, and plans for elected police commissioners.

According to Moore's analysis not only is "The localism of the Carswell/Hannan "direct democracy" movement is now good Coalition orthodoxy", but Cameron's policy guru, Steve Hilton, "enthusiastically lifted several bits of The Plan", the best-selling moderniser book written by Carswell and co-author Daniel Hannan.

[62] In January 2021 Carswell was appointed as president and CEO of the Mississippi Center for Public Policy, and left the UK.

[64] Carswell was portrayed by actor Simon Paisley Day in the 2019 HBO and Channel 4-produced drama Brexit: The Uncivil War.