Carr was born in Isleworth, London, England to Irish parents who were originally from Limerick. They then moved to Slough where Carr was educated. He holds dual citizenship with Ireland and the United Kingdom. He was educated at Burnham Grammar School in south Buckinghamshire and later at the Royal Grammar School, High Wycombe where he achieved 4 A grades at A-level, before going on to receive a 2:1 in Social and Political Sciences from Gonville and Caius College, Cambridge. After leaving Cambridge, Carr was hired by the oil company Shell as a marketing executive, a job which he described as "technically the easiest job on the planet". Less than two years afterwards, he took redundancy and joined JC Productions Ltd in order to begin a career in stand-up comedy.
2004 was a successful year for Carr's live work. In 2003 he sold out an entire month's performances of his Edinburgh Festival show Charm Offensive by the second day of the festival, and received 5-star reviews from four major newspapers. In 2004 he performed sold out solo shows at Dublin's Vicar Street, Leicester's Comedy Festival, Glasgow Festival, Kilkenny Cat Laughs and the Galway Festival along with appearances at The Bloomsbury Theatre where he filmed his first live DVD. Also in 2004 he threatened to sue fellow comedian Jim Davidson for using a joke that Carr considered 'his'. The matter was dropped when it became apparent that the joke in question was an old one used for decades by many different comedians, most notably the radio shows of Victor Lewis-Smith. He toured the country with his show, A Public Display of Affection, starting on 9 April 2005 at the Gulbenkian Theatre in Canterbury. He also appeared at the EICC during the Edinburgh Festival in August 2005 with his Off The Telly show. Later on in the year, in late November, he released his second DVD Jimmy Carr: Stand Up.