From 2021 to 2023, she played Christine Daaé in the West End production of The Phantom of the Opera, making her the first black performer to do so.

[6][7] St. Louis had her career breakthrough during her last year of Musical Theatre school in 2012, being cast in the ensemble of Ragtime.

In 2021 she became the first black actress to ever portray the lead role of Christine Daaé, in Andrew Lloyd Webber's The Phantom of the Opera.

[10] In 2023, St. Louis joined a star-studded cast for My Favourite Things, the 80th Anniversary Concert of Rodgers and Hammerstein at the Theatre Royal, Drury Lane.

Other performers included: Aaron Tveit, Audra McDonald, Joanna Ampil, Maria Friedman, Michael Ball, Daniel Dae Kim, Julian Ovenden, Marisha Wallace and Patrick Wilson.