O'Connell made her professional stage debut in the international revival of the show 42nd Street in 2016, in Théâtre du Châtelet, Paris,[5] forming part of the ensemble whilst understudying Peggy Sawyer.

In 2017, she moved back to the UK, performing in the same show in Theatre Royal, Drury Lane, forming part of the ensemble whilst understudying Anytime Annie.

[8][9] She left the show two weeks before its original closing date to rehearse for Hair at the Turbine Theatre, where she portrayed the role of Jeanie.

She starred in this revival alongside Six cast alumna Maiya Quansah-Breed, portraying the leading role of Mimi Marquez and Jocasta Almgill, as her partner Joanne.

[13] The creative team included director Arlene Phillips, choreographer Oti Mabuse and costume designer Gabriella Slade.

[14] August 2022 brought the original cast of SIX together to film a proshot, and Millie, reunited with her castmates, were granted the opportunity to reprise their roles once more at Hampton Court Palace in July.