Tunstall was born in Edinburgh, Scotland and was adopted eighteen days later by English parents. She had been born in Edinburgh to a half Chinese and half Scottish mother and an Irish father. KT has never met her biological father. Her adoptive father was employed as a physics lecturer at the University of St Andrews, and her adoptive mother a school teacher. Tunstall's family also included an older brother named Joe and a younger brother named Daniel.
Tunstall grew up in St Andrews, a town in Fife, attending Lawhead Primary, but spending her last year of high school in New England at the Kent School, a selective preparatory school in Kent, Connecticut. She spent time performing on Church Street in Burlington, Vermont, and at a commune in rural Vermont. Tunstall studied at the High School of Dundee in Dundee, Madras College in St Andrews, Kent School in Connecticut, and at Royal Holloway, University of London.
Throughout her twenties, she mostly played in independent bands including Elia Drew and Tomoko, and focused on songwriting, as well as performing with members of the fledgling Fence Collective including work in the Skuobhie Dubh Orchestra with King Creosote, who recently supported her UK tour. She also lived with Gordon Anderson (The Beta Band and The Aliens), whom the song Funnyman, on the album Drastic Fantastic, is about.She toured with the Klezmer band Oi Va Voi, and featured on their album, Laughter Through Tears. Her debut album, Eye to the Telescope, was released in late 2004. Tunstall's style of music varies from folk to pop. In Edinburgh and St Andrews, she played in a band called Red Light Stylus, which was regarded as one of the better bands to emerge from the limited Fife scene.
Shortly after the "Later" appearance, Eye to the Telescope was re-released and shot up the UK charts, eventually peaking at #3 (on its first release it had entered at #73); it was nominated for the 2005 Mercury Music Prize. It was released in the U.S. on 7 February 2006.
Tunstall's North American break came when American Idol contestant Katharine McPhee contacted her asking to use "Black Horse and the Cherry Tree" as her choice for a Billboard-themed week. At the time, the song was #79 on the Billboard charts. Tunstall had not been shy with her opinions regarding shows like Idol saying "The major problem I have is that it's completely controlled... they're told what to say. They're told how to sing." She chose to license the song as she felt that "no one on that show told Katharine McPhee to sing my song because no one knew it" Tunstall's belief was correct - the song was suggested to McPhee by Billboard columnist and author Fred Bronson. The song immediately jumped to #23 on the Billboard charts the week following McPhee's performance. She has later said "My status as a musician in America is pretty much cemented by Katharine McPhee, which is really interesting and funny for me because I've never been polite about how I feel about shows like that."
Tunstall released a new acoustic album in May 2006, KT Tunstall's Acoustic Extravaganza, which was first only available via mail order from her website. The album was re-released in stores worldwide in October 2006.
Tunstall sang with Scottish band Travis on their 2007 album "The Boy With No Name", on the track "Under The Moonlight", a song written by Susie Hug (late of Katydids).
Tunstall's third album, Drastic Fantastic, premiered on 3 September 2007 in Scotland, followed a week later on 10 September 2007 with the London release for the UK and 18 September 2007, in the US. In its first week, Drastic Fantastic reached #1 on the Scottish Album Charts, #3 on the UK Charts, and #9 on the U.S. Charts.