Tointon was born to parents Ken and Carol Tointon. Together with her actress sister, Hannah (born 1987), Tointon was brought up in Leigh-on-Sea. Both sisters attended St Michael's School, Leigh, and St Hilda's School, Westcliff-on-Sea, Essex. Tointon was diagnosed with dyslexia at age seven. She had speech and drama lessons at school. For about one year when she was a teenager, she dated James Bourne of the bands Busted, Son of Dork, and McBusted.
Tointon's first professional acting experience came when she played the part of Brigitta in The Sound of Music at the Cliffs Pavilion. Her first appearance on television was as an extra in EastEnders in 1994, when she played a school friend of Sonia Fowler. She played the part of student Pauline Young in 2001's first series of Teachers on Channel 4. She also starred in BBC's "Curriculum Bites" as a presenter in 2002.
Almost a month after her last Dream Team episode aired, it was revealed to the press that she would be joining the BBC soap EastEnders as a new character, Dawn Swann. Tointon auditioned for EastEnders at the same time as Tiana Benjamin, Matt Di Angelo, Kellie Shirley and Charlie Clements all auditioned for their respective roles. Tointon left EastEnders, along with co-star Ricky Groves, on 27 August 2009.
She co-starred with then partner Joe Swash in Snow White and the Seven Dwarfs in Bristol in the 2009/2010 pantomime season.
In July 2010, she recorded a documentary for BBC Three called Kara Tointon: Don't Call Me Stupid. The programme examined the impact dyslexia can have on people's lives and the difference different learning styles can have on people with dyslexia. Tointon revealed that she suffers from dyslexia, and has a reading age of 12. During the programme, Tointon visited Shapwick School in Somerset, which specialises in the education of pupils with dyslexia, and talked with the pupils about their experiences. Tointon was taught various line learning and organisational techniques by Dyslexia Tutor Claire Salter.
In 2008, Tointon won the Sport Relief special edition of Strictly Come Dancing with Mark Ramprakash. Her Samba was enough to get her the Glitterball Trophy.
In September 2010, it was announced that Tointon would be appearing as a contestant on Strictly Come Dancing Series 8, this time partnered with Artem Chigvintsev. The pair achieved good scores in the first five weeks of 30, 32, 31, 32 and 37. In week 5, Tointon was awarded the first 10 of the series by Alesha Dixon for her Pasodoble. In week 6 she was top of the leaderboard with her Salsa. She also had good scores for the next five weeks of 36, 38, 35, 34 and 38. In week 11, she got her highest score of 39 for the Viennese Waltz and Rumba and got the maximum 5/5 for the Swing-a-thon. Tointon made it to the final along with Matt Baker and Pamela Stephenson, where she made it to the final two, then went on to win and be crowned champion beating Matt Baker and his partner Aliona Vilani.
In May 2011, Tointon played Eliza Doolittle in the West End production of Pygmalion at the Garrick Theatre. She received fantastic reviews for the Philip Prowse production of the well known play which opened on 25 May and ran until 3 September. Her co-stars were Rupert Everett and Dame Diana Rigg.
Kara played what the Guardian’s Michael Billington called “a luminous and refreshing” Eliza to Rupert Everett’s Professor Higgins “capturing perfectly the arc of [her] development”. Whatsonstage similarly picked up on the spirited chemistry the leads seem to share; “[Kara’s] remarkably assured on the stage, blessed with malleable good looks and tremendous lung power, and she makes Eliza a truly feisty opponent to Everett’s self-obsessed, grandiloquent Higgins.”
In the Telegraph, Charles Spencer celebrated Kara’s “warmth and vulnerability” marking her out as “an actress of truly great potential” while the Daily Express's Paul Callan said “ the role of Eliza is a challenge for any actress and Miss Tointon faced it with skill and charm”.
In 2012 Kara was cast as Evelyn in Alan Ayckbourn's west end revival of Absent Friends, a play he wrote and set in 1974. She appeared along with Reece Shearsmith, Katherine Parkinson, Steffan Rhodri, Elizabeth Berrington and David Armand at the Harold Pinter theatre, a Sonia Friedman production directed by Jeremy Herrin. Michael Billington of The Guardian wrote "all the performances in this production are finely judged. Kara Tointon has just the right broodiness as the edgy Evelyn, who is like a comic version of the young mother in Edward Bond's Saved".
She then played Giny in Ayckbourn's Relatively Speaking, a four handed comedy set in 1965 with Felicity Kendal, Jonathan Coy and Max Bennett at the Wyndham's theatre in the summer of 2013. Tim Walker for his review in The Telegraph said "Tointon, for her part, gives an intelligently nuanced performance as the beautiful, but profoundly flawed Ginny... There are few more exciting actresses now working on the West End stage".
In August 2009, she was unveiled as the new face of Michelle for George underwear at Asda.
In February 2011, Tointon took part in the BT Red Nose Desert Trek for Comic Relief 2011.
In September 2011 it was announced that Kara would play a lead role in the new film 'Last Passenger' acting opposite Dougray Scott and directed by Omid Nooshin.
Kara played the part of "Maria" in "The Sound of Music Live" in the UK, broadcast on 20 December 2015.
From November 2017, Kara will play the role of Olivia in The Royal Shakespeare Company production of Twelfth Night.
In September 2014 Tointon took part in the Great North Run to raise money for Share A Star, a charity set up to help severely unwell children and teenagers.