Nagasawa began her acting career at age twelve, when she starred as a young psychic orphan in the science fiction film Pyrokinesis (2000).

In 2007, Nagasawa was nominated for the Japan Academy Film Prize for Best Leading Actress and attained a Bunshun Raspberry Award for playing the title character in Tears for You (2006).

After playing a supporting character in Makoto Shinkai's Your Name (2016), she portrayed the protagonist in the critically acclaimed science fiction drama Before We Vanish (2017), for which she won the Mainichi Film Award for Best Actress.

[6] After winning the competition, Toho instructed director Shusuke Kaneko to give her a role in his science fiction action film Pyrokinesis.

Nagasawa's performance in Crying Out Love in the Center of the World received both a Blue Ribbon Award[9] and a Japan Academy Prize for Best Supporting Actress.

Kiyoku Yawaku (Beyond the Memories), a live action adaptation of Ryo Ikuemi's manga series, was screened at the 17th annual Japanese Film Festival in Melbourne, Australia in 2013.

[18] Umimachi Diary (Our Little Sister), a live action adaptation of Akimi Yoshida's manga, competed for the Palme d'Or at the 2015 Cannes Film Festival.

Her performance in Proposal Daisakusen (Operation Love) received the most votes in the Nikkan Sports Drama Grand Prix Best Actress category.

In 2009 she played a supporting role in the NHK taiga drama Tenchijin, and in 2010 she joined several other leading Japanese actors in the Fuji TV 50th anniversary mini-series Wagaya no Rekishi.