Jamie Cullum was born in Rochford, Essex, on August 20, 1979, and grew up in Hullavington, Wiltshire. His father was born in Jerusalem, and his mother's family came from Burma and India and settled in Wales. He attended Grittleton House School and then Sheldon School in Chippenham, and went on to the University of Reading, where he read English Literature and Film Studies and graduated with first-class honours. He has said he failed his grade 4 piano exam and can barely read music, which fits the way he came up: through playing rather than through conservatory training.

His first album, Heard It All Before (1999), was made for about 480 pounds in a run of 500 copies and billed to the Jamie Cullum Trio, with Raph Mizraki on bass and Julian Jackson on drums. It is a set of ten standards, from Cole Porter's "Night and Day" and Duke Ellington's "Caravan" to Billie Holiday's "God Bless the Child." Through his early years he played PizzaExpress restaurants in London three or four times a week, and that circuit, plus a guest spot on bassist Geoff Gascoyne's album Songs of the Summer, led to his first label release. Pointless Nostalgic (Candid, 2002) paired standards with arrangements of Thelonious Monk's "Well You Needn't" and Bob Dorough's "Devil May Care," and included a piano-and-voice reading of Radiohead's "High and Dry." Gascoyne played bass and Sebastiaan de Krom drums, with a horn section that included Ben Castle on tenor saxophone. It reached No. 55 in the UK and went Gold.

Television did the rest. After his first appearance on Parkinson in April 2003, Universal signed him to a reported three-album contract worth 1 million pounds, beating a bid from Sony. Twentysomething (October 20, 2003), produced by Stewart Levine and recorded in London at Mayfair Studios in the summer of 2003, mixed jazz standards such as "What a Difference a Day Made" and "I Get a Kick Out of You" with songs by Jimi Hendrix ("The Wind Cries Mary"), Jeff Buckley, and originals he wrote with his brother Ben Cullum. It peaked at No. 3 in the UK and has sold about 2.5 million copies worldwide. He was nominated for Best British Breakthrough Act at the 2004 Brit Awards, won Best Jazz Act at the 2004 MOBO Awards, and was nominated for a Grammy for Best Jazz Vocal Album the following year for the record.

Catching Tales (2005, UK No. 4) was again produced by Levine, with hip-hop producer Dan the Automator working on the first single, "Get Your Way." A live set from Ronnie Scott's followed in 2006, and in 2007 he won the Ronnie Scott's Jazz Awards prize for Best British Male. He played the Hollywood Bowl with the Count Basie Orchestra and worked through a long festival circuit that included Montreux, Montreal, Glastonbury and Coachella. The Pursuit (November 10, 2009), produced by Greg Wells and recorded partly in Los Angeles and partly at his own Terrified Studios in Shepherd's Bush, was a looser pop record with a Cole Porter opener. His most visible song of the period came from a film: he co-wrote the title song for Clint Eastwood's Gran Torino (2008), a duet-style closing-credits track on which Eastwood also sings, and it was nominated for the Golden Globe for Best Original Song. In July 2009 he also appeared at the Montreux Jazz Festival with the CTI All Stars, the touring edition of Creed Taylor's label, and is credited with vocals on "Use Me" on the live album issued in 2010.

In April 2010 he began presenting a weekly jazz programme on BBC Radio 2, which won him a Best Jazz Format award at the 2012 New York Festival Radio Awards, Jazz Broadcaster of the Year at the 2012 Parliamentary Jazz Awards and Best Music Programme at the 2014 Radio Academy Awards. He married the author Sophie Dahl in January 2010, and the couple have two daughters. His 2012 Eurovision connection came as a songwriter: he co-wrote "Standing Still," Germany's entry sung by Roman Lob, with Steve Robson and Wayne Hector. Momentum (May 20, 2013, Island, UK No. 20) was produced by Dan the Automator and Jim Abbiss and was a pop-leaning record, with covers of "Love for Sale" and "Pure Imagination" among originals. Interlude (October 6, 2014), his clearest return to jazz, was produced by Benedic Lamdin of Nostalgia 77 and was shaped by his radio show, with duets alongside Gregory Porter and Laura Mvula. In April 2016 he played at the White House as part of the International Jazz Day global concert. Taller (June 7, 2019, Island, UK No. 17), co-produced with Troy Miller, went back to mostly original songwriting.

His most recent studio album is The Pianoman at Christmas (November 20, 2020, Island), original Christmas songs recorded in Studio 2 at Abbey Road with 57 musicians and produced with Greg Wells, which reached No. 11 in the UK; an expanded Complete Edition followed in November 2021. He continues to present the Radio 2 jazz programme, which lists new episodes on Tuesday evenings, and as of late October 2026 two Royal Albert Hall nights are listed for December 10 and 11, 2026.

Cullum is a singer-pianist whose catalog sits openly across two worlds. One is the jazz standards repertoire (Porter, Gershwin, Monk, Dorough, Dietz and Schwartz), which fills Heard It All Before and Pointless Nostalgic, runs through much of Twentysomething and is the model for the covers on Interlude. The other is pop and rock: he built his early reputation partly on covers of Radiohead, Jimi Hendrix, Jeff Buckley and Pharrell Williams songs, he has played Kanye West and The White Stripes numbers in concert, recorded The Killers' "The Man" for a film soundtrack, and since The Pursuit a large share of his albums is original pop and singer-songwriter material. Momentum and Taller are better described as pop records with jazz harmony and piano in them than as jazz albums, and the song he is best known for outside Britain is a film theme. He is generally described as a crossover artist whose roots are in jazz, and he names Miles Davis among the music he listens to.

His live show uses a stomp box under his foot, a looping machine and beatboxing, which he applies to faster songs such as his versions of "Seven Nation Army" and "Gold Digger." Critical reception has been divided in the usual crossover way. John Fordham praised his ability to learn fast and listen hard on Twentysomething, while Robert Christgau questioned what the swinging, pop-minded approach added artistically. AllMusic rated Catching Tales, Interlude and Taller positively, and one Associated Press review of Taller called it stylistically scattershot. His radio programme has also made him a promoter of other artists: Gregory Porter and Laura Mvula both reached a mainstream audience partly through it, and both appear on Interlude.