Tokyo-born Mitsuishi graduated from high school in 1986 and then attended the Katsuta Seiyū Academy. While studying, she worked part time as an elevator girl in the Sunshine 60 building. Afterward, she found a job as an office lady but had to quit due to taking too much time off. In 1989, Mitsuishi made her seiyū debut as Tomoyo in the OVA Ace o Nerae! Final Stage. She gained instant fame with her role as Usagi Tsukino when Sailor Moon debuted in 1992 and again with her portrayal of Misato Katsuragi in Neon Genesis Evangelion. Mitsuishi is widely regarded as one of the most influential seiyū in the industry, and her interest in the Ebichu animated adaptation helped produce the project. She is married with a daughter and enjoys riding a Yamaha FZ250 Phaser motorcycle. Currently, Mitsuishi is a freelancer and was once part of the talent management firm Arts Vision.