Marsh was born in Royton, in the Metropolitan Borough of Oldham, Greater Manchester, England, and attended Crompton House School.
She was interested in singing, acting and modelling from a young age, and initially appeared as an extra on soap operas such as Brookside. She later entered the TV sound-alike contest Stars in Their Eyes as Emma Bunton and competed in ITV's singing competition Popstars, before focusing on modelling. In 2002 she became a Page Three girl for The Sun newspaper, and in 2003 she started a successful modelling partnership with Lucy Pinder.
She has appeared in many lads' mags such as Loaded, Perfect 10, Nuts, Maxim, Zoo and Playboy. In the UK FHM 100 Sexiest Women poll 2005 Marsh was a new entry and finished 33rd overall. She finished in 47th in the 2006 poll. She has a tattoo of a religious cross with angel's wings bearing the word "Blessed" written across the middle on her lower back. It often appears in her current magazine shoots.