Hiiragi began her career as a child actress while at the age of six, appearing in numerous commercials. In 1999, she appeared in the Nippon Hōsō Kyōkai asadora Suzuran, portraying the main character, Moe Tokiwa. In 2001, she worked on Hayao Miyazaki"s award-winning anime film Spirited Away, in which she voiced the film"s protagonist, Chihiro, for which her performance was praised.
She returned to Ghibli as a voice in their films, Ponyo on the Cliff by the Sea and From up on Poppy Hill.
In 2002, she appeared in the high school baseball television program Netto Koshien as a field reporter. In 2005, she appeared in the NTV program Nobuta o Produce, portraying the character Kasumi Aoi.