Hara started her career as a junior idol under the name Mai Nanami. As a teenager, she sang the theme song Spicy Days (スパイシーデイズ, Supaishiideizu) for the animated TV show The Marshmallow Times on TV Osaka in 2004. She also had parts in two movies in 2005, the March TV drama The Dream of Delinquent Boys (不良少年の夢, Furyō Shōnen No Yume) for TBS, and the April theatrical release, horse-racing melodrama Haru Urara (Lovely Fields), which was later released as a DVD.

She had also made a bikini-model photobook, Mai Nanami First Photobook, in May 2005 and a non-nude (gravure) DVD titled Mai Nanami: Yamagishi Shin Digital Movie Museum which was released in August 2005. As Nanami, she also played the female lead in another mainstream movie that was due to be released in Japan in 2006 but had its debut at the American Film Market in Santa Monica, California in November 2005 as Deep Sea Monster Reigo. The film was eventually released in Japan as Reigo: The Deep Sea Monster vs the Battleship Yamato.

After a long hiatus, she re-emerged as Saori Hara in August 2008 and appeared in a non-nude gravure video titled Clear Water. The next month, in September, she posed nude for the first time in the Japanese men's magazine Sabra, and it was announced that she was contracted to the Soft On Demand (SOD) adult video studio. She became the face of the SOD group in its anti-STD campaign in November 2008, a role formerly played by long-time SOD AV Actress Nana Natsume. Hara's first adult video (AV) was released by SOD in January 2009 with the title Real Celebrity Saori Hara: Miraculous AV Debut and was reported to have sold 100,000 copies.

On December 11, 2009, Shueisha published Hara's autobiography titled My Real Name Is Mai Kato: Why I Became an AV Actress (本名、加藤まい　私がAV女優になった理由) (ISBN 4087805409). The 144 page book details Hara's upbringing and family life, her views on sex and her career as a teen idol and eventual AV idol. Her book is one of a number of autobiographical publications by actresses about the AV industry going back to Ai Iijima's million-selling Platonic Sex and including Mihiro's partially fictional May 2009 work nude and Honoka's tell-all Biography of Honoka: Mama, I Love You published in January 2010, which adult media reporter Rio Yasuda sees as marking a trend in which the AV industry is losing its stigma and being assimilated into Japanese popular culture.

According to a 2010 article in Shukan Post, Hara and Maria Ozawa were the two most downloaded AV actresses in China and in August 2010, it was announced that Hara was cast with fellow SOD Actress Yukiko Suo (周防ゆきこ, Suō Yukiko) for the US$3.2 million 3-D Hong Kong erotic film 3D Sex and Zen: Extreme Ecstasy produced by Stephen Shiu who was the executive Producer of the Category III film Sex and Zen. The film opened in April 2011.

In 2011, Complex magazine ranked her at #19 in their list of “The Top 50 Hottest Asian Porn Stars of All Time.”

Matsunoi appeared in the April 21, 2014 issue of Weekly Playboy in a nude photo spread under the headline "Rebirth."

In 2015, Matsunoi was cast along Masahiro Inoue in the television series Garo: Gold Storm Sho.