Henshall's first performance will be 12 May 2014, nine years to the day since the show officially opened at the Victoria Palace Theatre.
Also joining the cast in May is Chris Grahamson who will play Billy's brother Tony and Barnaby Meredith returns to play Older Billy.
Ann Emery will continue to play Grandma, a role she created on stage when the show premiered in 2005, and Deka Walmsley will continue as Billy's dad.
Henshall's myriad West End credits include Chicago, The Woman in White, Marguerite, Les Misérables, Oliver!, Crazy For You, Cats, Miss Saigon and She Loves Me, for which she won an Olivier Award.
She was last on stage in Thea Sharrock's revival of Blithe Spirit at the Apollo Theatre in 2011.
Based on the hit 2000 film and set against the mining strikes of the 1980s, Billy Elliot - The Musical recounts the tale of a motherless boy whose father wants him to learn to box but who instead discovers a love for ballet that leads him from secret lessons to a place at the Royal Ballet School.