Dixon was born in Welwyn Garden City, UK, one of seven children born to a British mother and Jamaican father. She was educated at Monks Walk School, Welwyn Garden City, Hertfordshire, and had planned to become a PE teacher after leaving college. After completing a diploma course, she had planned to take up a place at Loughborough University when she was approached by a record producer, who asked her if she could sing and if she was in a band. She then decided to put her degree on hold.

Her career began in 1999 when she met Sabrina Washington. They both joined Dance Attic (a popular hip-hop/jazz dance academy in West London) and together they formed a duo and recorded a demo titled "Inspiration". During an audition, they met with Su-Elise Nash and they formed a trio. They were soon signed to Telstar Records with the addition of a new member, Zena McNally, and became the popular UK garage/R&B girl band, Mis-Teeq.

Mis-Teeq released their debut single "Why?" in 2000, and it was not long before McNally left the band to pursue her own solo career. The band made a decision to remain a trio. In 2001, they released their debut album Lickin' On Both Sides, featuring hits "All I Want", "One Night Stand", "B With Me" and double A-side single "Roll On/This Is How We Do It". After the success of their debut album, 2003 saw Mis-Teeq becoming the faces of JD Sports stores nationwide.

In 2003, Mis-Teeq released their second album Eye Candy, which included the singles "Scandalous", "Can't Get It Back" and "Style". The last song Dixon recorded with Mis-Teeq was "Shoo Shoo Baby" for the Disney film Valiant. In March 2005, their Greatest Hits compilation was released and it was announced that they were separating to pursue solo careers.

Dixon signed a £500,000, three-album deal with Polydor Records. She spent a year writing and recording her solo debut album, Fired Up, working with a wide range of producers including Richard X, Xenomania, Johnny Douglas, Brian Higgins, Estelle and Paul Epworth.

In June 2005, she announced her first solo single to be "Superficial". However, at the last minute, the song "Lipstick" was chosen as the first single. "Lipstick" was released on 14 August 2006, and charted at 14 on the UK Single Charts. She released her second single "Knockdown" on 30 October 2006, which charted at 25 on the UK Download Chart.

On 6 November 2006, it was revealed that Dixon had parted company with Polydor Records. Then she signed a four-album contract with Asylum Records in 2008. Dixon's first new album as part of her contract, The Alesha Show was released in the UK on 24 November 2008. The official first single from the album, "The Boy Does Nothing" became her first solo top ten hit on the UK Singles Chart, following a 76 spot jump from 84 to 8 on the UK Singles Chart on downloads alone. The second single from The Alesha Show was "Breathe Slow" and entered the UK Singles Chart at number thirty-nine on through solely download sales. The single rose for three weeks before breaking into the top ten at number six, giving Dixon her second top ten hit in the UK from downloads alone, and eventually peaked at number 3 on the Top 40 UK Charts.

As a model, Dixon is represented by Select Model Management and has appeared on the cover of Cosmopolitan, FHM, Arena, Company, Bliss, Another magazines and in various other Editorial fashion shoots. Dixon was cast as the lead female in the video clip for "She Wants to Move" by N*E*R*D after Pharrell spotted her on the cover of Arena magazine. Dixon has been featured in FHM's annual 100 sexiest women in the world poll on a number of occasions. In March 2007, she was announced as the face of the urban clothing brand Ecko Red. She was selected to front a promotional campaign as the face and legs of Veet in 2008.