Born in Totton near Southampton in 1981, Kelly moved to a farm on the Isle of Wight with her family when she was nine years old. After putting on shows for her friends, she made her first musical appearance in a middle-school production of Bugsy Malone. Following training at the Shanklin Stagecoach of Performing Arts School, Kelly traveled to London at the age of 16 with her mother in order to gain experience in professional open auditions. Originally landing the part of the understudy for Belle in Beauty and the Beast, she soon went on to become the leading lady in several West End productions including Mamma Mia! and My Fair Lady, and also made her Broadway debut in Fiddler on the Roof. But it was as the lead role in Mary Poppins where she attracted the most attention, featuring in over 400 performances and winning an Olivier Award for Best Actress in a Musical in 2005.

A year later she signed to EMI offshoot Angel Records, where she released The Storm Inside, a collection of original tracks and interpretations of songs from the likes of the Cardigans, Nick Drake, and Paul Weller. Produced by Marius de Vries, it featured collaborations with Jamie Cullum, Guy Chambers, and the Divine Comedy's Neil Hannon, and was later re-released with three brand-new tracks under the new title What's It All For. Since then, she has returned to the stage, playing the role of Galadriel in The Lord of the Rings; has performed at the BBC Proms with Michael Ball; and was the special guest in Jools Holland's Rhythm & Blues Orchestra nationwide tour. In 2007 she played Lucy Barker in Tim Burton's adaptation of Sweeney Todd. ~ Jon O'Brien