Hazell was born in Lewisham, London, England, grew up in Grove Park, and attended the Ravensbourne School in Bromley. Her mother, Amber, is a dinner lady and her father, Roy, is a window fitter; they separated when she was thirteen. As a teenager, Hazell was a frequent truant and stayed out late on school nights drinking. She confesses to having only taken five GCSE examinations due to being ejected from several of them; however, she did very well in mathematics and English.

At sixteen years of age, Hazell left school to work as a hairdresser. Her colleagues persuaded her to try her luck at modeling (she later quit hairdressing) and she enrolled at Lewisham College to start a City & Guilds in fashion, but was not at the college for very long. At seventeen, she competed in The Daily Star's "Search for a Beach Babe" contest and won. A friend told her about The Sun's Page 3 Idol competition. Despite some initial uncertainty about entering the contest, she submitted some photos. She was eventually chosen the winner in December 2004. Three cast sculptures were made of her bust by sculptor Leigh Heppell. Two are in the Sun offices and one is in her house. Another part of Hazell's Page 3 Idol win was a one-year exclusive glamour modeling contract with The Sun. She is currently being handled by Jon Fowler.

Hazell regularly featured in Zoo until 2008, and continues to appear in The Sun at least once every two weeks. She has been on the cover of The Sun's 2006 and 2007 Page 3 calendars, in addition to her own 2006 and 2007 wall calendars, the latter of which sold 30,000 in its first few days of release. In April 2008, Hazell left Zoo and signed an exclusive deal to appear in their rival magazine Nuts.

She is the face of Sony Computer Entertainment Europe's Formula One 06 video game for the PlayStation 2, PlayStation Portable, and F1 CE for PlayStation 3.

Keeley is the presenter of Byte Me TV, alongside IT expert Gary Schwartz. Byte Me TV is an online program that explains technology in an easy to understand way.