Karen Jane Wallace was born at 204 Durants Road, Enfield, North London. After schooling, Wallace trained as a make-up artist and worked at the Royal Shakespeare Company for two years. She met and became friends with actor Iain Glenn, who helped her apply for London drama college The Poor School, from where she graduated in 1999.

While undertaking auditions - including for Andrew Lloyd Webbers Sunset Boulevard, she worked as a real life barmaid at her local pub, The Royal Oak in Loughton, Essex.

Her first television appearance was in an episode of the ITV police drama series The Bill in 1999. Wallace is best known for playing the role of Kat Slater in BBC One soap opera EastEnders between 2000 and 2005. In Summer 2008, Wallace said she hadn't ruled out a return to EastEnders.

Since leaving EastEnders in November 2005, Wallace has made a guest appearance, as Kat Slater, in the 2005 French and Saunders Christmas special. On February 17, 2006, Wallace was the "guest host" of an edition of the Channel 4 programme The Friday Night Project.

She has recently been seen in the one-off BBC1 television dramas A Class Apart opposite Nathaniel Parker, The Dinner Party with Lee Evans and Alison Steadman. After visiting Hat Trick Productions with her agent, and singing an impromptu version of Marie Lloyd's song The Boy I Love Is up in the Gallery, she was offered the lead role in Miss Marie Lloyd - Queen of The Music Hall for BBC Four, opposite Stephen Fry's Oscar Wilde.

Wallace made her West End theatre debut, replacing Denise van Outen in the role of "Maureen" in Rent remixed at the Duke of York's Theatre from December 27, 2007 until February 2, 2008. Wallace played Amy Kriel in the third series of ITV1's Wild at Heart, and will be next seen in a run of Haunted at the Arts Theatre from May 24.

It has also been reported that Jessie is set to play Ellie in a new BBC1 drama called Hope Springs about four ex-cons hiding out in a village.