Stenson joined the production in October last year as supporting character Margot, but will now replace departing actress Susan McFadden as main character Elle Woods.
The actress will be joined in her run by Lee Mead, who is to replace Alex Gaumond as Woods's main love interest Emmett Forrest from June 20.
Stenson and Mead will round out a returning cast including the likes of Nicola Brazil, Emma Bateman, Natalie Casey, Peter Davison and Siobhan Dillon.
Stenson is best known for playing Steph Cunningham on the Channel 4 soap Hollyoaks for ten years, before announcing her decision to quit the show in June 2010.
The actress made her final regular appearance on the soap in November last year, and previously admitted that she had always wanted to pursue a career in musical theatre.
Stenson will take over the role of Elle Woods in Legally Blonde, The Musical from July 11.