Saki first joined Hello! Project in 2002 as one of the fifteen children chosen from the Hello! Project Kids auditions after performing “Dekkai Uchuu ni Ai ga Aru” by Morning Musume. In 2004 Berryz Koubou was formed, with the intention of rotating the kids throughout the unit. Nakajima didn’t make the original pick, and the idea was eventually dropped.
The remaining kids ending up forming ℃-ute in 2005. The group didn’t make its official debut until late in 2006, with their first official single released in February 2007. Nakajima is also a member of Gatas Brilhantes H.P., the Hello! Project futsal (indoor soccer) team.
In October 2007, Nakajima was placed in the unit Athena & Robikerottsu along with fellow ℃-ute member Okai Chisato, as well as Niigaki Risa and Mitsui Aika of Morning Musume.
In 2009 she was chosen along with fellow Hello! Project Kids members Kumai Yurina and Sugaya Risako as well and Mitsui Aika from Morning Musume to be in a newly formed group Guardians 4 formed to sing the openings to the anime Shugo Chara!. She was later named leader of Guardians 4
Nakajima was chosen to be in the revived unit Petitmoni V along with fellow ℃-ute member Hagiwara Mai and soloist Mano Erina.
During the ℃-ute 2010 concert, Nakajima dislocated her hip and was taken out of ℃-ute activites until she had fully recovered.
Alongside Morning Musume member Niigaki Risa and Hello! Pro Kenshuusei members Takagi Sayuki and Sato Ayano, Nakajima will be starring in the 2010 horror movie Gekijouban Hontou ni Atta Kowai Hanashi 3D.
On July 14th, she opened her Ameba blog tited “Blog day’s”.
On August 7th, it was announced That Nakajima will be in a movie tittled Zomvideo. The movie will be release in 2012
Nakajima will be in a stage play featuring Okai Chisato, Miyamoto Karin and Kudo Haruka. titled 1974 Ikunayo The play will run from 12/14~18.
On July 20, 2012, it was announced that Nakajima, Yajima Maimi, Tokunaga Chinami, Natsuyaki Miyabi, and Iikubo Haruna were chosen to form the unit DIY♡.
On July 25 it was announced that ℃-ute’s Nakajima Saki, Yajima Maimi, Hagiwara Mai and Berryz Koubou’s Shimizu Saki, Sudo Maasa, Kumai Yurina and Sugaya Risako will be performing in an stage play titled Cat’s Eye based on a manga. The stage play will run from 9/22 till 9/30.