She was born in Great Yarmouth in Norfolk, the youngest of three children and is the younger sister of former model and fashion designer Tanya Spearritt. Spearritt has German-American, Irish and Canadian ancestry. At the age of 3, Spearritt modeled in a Mothercare catalogue. When she was 12, she landed a part in the Great Yarmouth Amateur Dramatic and Operatic Society's production of Annie. Thereafter she successfully auditioned for the National Youth Music Theatre, and became part of the company. While with the NYMT she met future S Club member and boyfriend Paul Cattermole, and her friend and future flat mate, actress Sheridan Smith. She appeared in the NYMT's productions Pendragon (1994–1995) — performing in New York and Hong Kong, Tin Pan Ali (1996), and the West End musical Bugsy Malone (1997) — she was a member of the original cast.

In 1998 she got a part in the television film The Cater Street Hangman, playing the part of a maid who gets murdered. She had also made appearances on the National Lottery Show and Blue Peter. While Hannah was studying mathematics, performing arts and media studies at college in Great Yarmouth, she auditioned for S Club 7 after seeing an advert in a magazine - it was a life changing decision.

She was a member of S Club from 1999 to 2003. She co-starred in the groups own TV series - Miami 7 (1999), L.A. 7 (2000), Hollywood 7 (2001) and Viva S Club (2002). She took part in three UK tours, won two prestigious BRIT Awards and numerous other awards.

In the last week of S Club's existence, Spearritt management claimed she was at home in bed with the flu. She was actually in Los Angeles to audition for a role in the film Agent Cody Banks 2: Destination London, a fact that Hannah revealed at the UK premiere of the film on 2004-03-24 in London. She got the role and three days after leaving the band she began filming.

In February 2006, it was revealed that Hannah had landed a lead role in a brand new ITV series called Primeval. Hannah plays the role of Abby Maitland, a reptile enthusiast who gets mixed up with time travelling Professor Nick Cutter (Douglas Henshall) when searching for a species of reptile she had never encountered. The series began on February 10, 2007.