While studying for her A-levels, she auditioned and joined London's West End production of Miss Saigon. She later went on to play Kim and became the first Japanese national to play a lead role in the West End. From 1993 to 1994, Mori had a regular role on the hospital drama Casualty as the hospital receptionist Mie Nishikawa. Film roles followed, including appearances in the 1997 musical comedy film Spice World and Topsy-Turvy (1999). Mori also appeared in the television programmes Thief Takers (1997), Judge John Deed (2001), Spooks (2002), Mile High (2003) and Powers (2004). In 1995, she had a small role as a Japanese computer hacker in the film Hackers. Mori had a major role in the 2005 BBC docu-drama Hiroshima, which contained dramatic re-enactments of the 1945 atomic bombing. Mori also provided the voice acting for the villainess Mai Hem for the game Perfect Dark Zero.
In 2005, Mori had a small role as Dr. Sato in the Doctor Who episode "Aliens of London". Producer Russell T Davies took note of her performance and decided to bring the character back as a regular in the first two series of the Doctor Who spin-off Torchwood. Mori's Torchwood character, Toshiko Sato, was killed in "Exit Wounds", the final episode of Torchwood's second series, but has not ruled out the possibility of returning to the show at a later date. She returned to the West End on 20 November 2006, when she took over the role of Christmas Eve from Ann Harada in the London production of Avenue Q, a role which she played until 14 April 2007. Mori portrayed Yoko Ono in the BBC Four production Lennon Naked, which was broadcast in the UK on 23 Wednesday June 2010. The film reunited her with Christopher Eccleston, who played John Lennon, opposite whom she played Dr. Sato in her Doctor Who appearance. Before joining Torchwood, Mori acted as Kim in Miss Saigon alongside Future co-star John Barrowman as Chris.
Mori appeared in the movie Everest (2015), as Yasuko Namba, opposite Jason Clarke, Josh Brolin, John Hawkes, Jake Gyllenhaal, Robin Wright, and Keira Knightley.
