Masami Nagasawa was born on June 3, 1987 in Iwata, Shizuoka. She is the eldest daughter of Kazuaki Nagasawa, former manager of the J. League team Júbilo Iwata. She is a graduate of Horikoshi High School.

In 2000, at the age of twelve, Nagasawa joined the 5th Toho Cinderella Audition. Out of 35,153 applicants, she was selected as the winner and made her entrance into show business. She made her film debut that year, appearing in the film "Crossfire." She also began appearing in magazines.

In 2003, Nagasawa landed her first starring role in the movie "Robocon." For this role, she earned the title of Rookie of the Year in the 27th Japan Academy Awards. That same year, she also appeared in "Godzilla: Tokyo S.O.S." with Chihiro Otsuka, who won the Judges Prize at the same Cinderella Audition. Together, they sang "Mothra's Song" for the movie's soundtrack.

Nagasawa's big break came in 2004 when she played the heroine in the hit movie "Crying Out Love, In the Center of the World." She received several awards for the role, including Best Supporting Actress at the 28th Japan Academy Awards.

In 2006, Nagasawa got her first lead role in a television drama series, a remake of the 1981 cult classic film "Sailor Suit and Machine Gun." Under the name of the main character (Izumi Hoshi), she sang the theme song for the series. It was later released as her first single.