Jean Simmons took on the role of the older Sophie Hatter in the English-language dub of the Studio Ghibli film Howl's Moving Castle, released in 2004. In this production, she voiced the character after Sophie is cursed to live in the body of a ninety-year-old woman, while actress Emily Mortimer voiced the young Sophie. The English dub featured a cast that also included Christian Bale as Howl and was directed by Pete Docter.

Simmons was a highly acclaimed British actress with a career stretching back to the 1940s, transitioning to voice acting in her later years. Her entry into anime and animation voice work occurred during a period of semi-retirement, where she took on select projects. Beyond Howl's Moving Castle, her major voice roles include a council member in the computer-animated science fiction film Final Fantasy: The Spirits Within released in 2001, and a character named Shepway in the animated feature Thru the Moebius Strip, which was in production around the same period. Her voice acting credits are limited compared to her extensive live-action filmography.

Regarding collaborations, her work on Howl's Moving Castle was her first encounter with the films of director Hayao Miyazaki. She did not work directly with Miyazaki on the English dub, but she later spoke highly of his imagination and demeanor. Her primary creative collaboration for this project was with Pete Docter, who supervised the English-language adaptation.

Her notable achievements in this specific field include bringing authenticity to the aged version of Sophie, a role she described as a meditation on aging. As an actress, Simmons had received Academy Award nominations, a Golden Globe, and an Emmy Award for her live-action performances prior to her voice work. Her involvement in Howl's Moving Castle represents a significant instance of a classic Hollywood star participating in an acclaimed anime production. She continued to work until her death in 2010 at the age of eighty.