Miyazaki started working in the entertainment industry at the age of four. Initially she appeared mostly in commercials, magazine advertisements, and as an extra in various television dramas. Miyazaki made her film debut in Ano Natsu no Hi at the age of fourteen.

When Miyazaki was 14, she began to draw international attention for her role as the survivor of a traumatic bus hijack in Shinji Aoyama's Eureka. The film won the FIPRESCI Prize at the Cannes Film Festival 2000, and resulted in her receiving the Best New Actress Award of the Japanese Professional Movie Awards. In 2003, she also made her musical debut in The Little Prince.

Later, she won Best Actress Award in the Cinemanila International Film Festival for her performance in Harmful Insect. She teamed up with Aoyama again in Eli, Eli, Lema Sabachthani?, an Un Certain Regard selection at Cannes 2005. Later in the same year, Miyazaki co-starred with Mika Nakashima in the mainstream and commercially successful Nana.

In 2008, she won the Galaxy Individual Award for her performance in the NHK drama Atsuhime.

Beyond acting, Miyazaki has become a much sought-after spokesperson and model. She has appeared in commercials for major corporations like Aflac, Tokyo Metro and Olympus. In early 2008 Miyazaki was also selected as Emperio Armani's new print advertisement model.

With each year Miyazaki has taken a more prominent position in activism projects. In 2005, Miyazaki with older brother Masaru Miyazaki traveled to India to experience firsthand poverty stricken in that area and highlight those problems to their fans via their 2006 photobook Tarinai Peace. In 2006, the siblings traveled to Denmark and Finland to investigate global warming. Their experiences were then published in their 2007 photobook Love.Peace & Green Tarinai Peace2. More recently, in April, 2008, Miyazaki took part in the Gold Ribbon Walking event in Roppongi, Tokyo to raise awareness and funds for childhood cancer. Her 2008 film Children of the Dark also addresses issues of child exploitation.