In 1987, Nakai entered the Japanese talent agency Johnny & Associates at the age of 14. He is the second person after Noriyuki Higashiyama that was allowed to skip auditions and directly enter the agency as a trainee, thus becoming known as one of the elite in Johnny's.
Fans have expressed that "Nakai's inability to sing is somewhat of a national joke today in Japan". Regardless of his lack of ability, Nakai has found eventual major success with SMAP as the group went on to become one of the most successful boy bands in J-pop history.
In 2007, Nakai was given his first movie role in nearly five years, portraying a humble barber wrongfully sentenced to death as a war criminal in the movie Watashi wa Kai ni Naritai(私は貝になりたい,I Want to Be a Shellfish?).
In 2009, Nakai was given his first lead role in a drama that aired on Fuji TV's Monday 9:00 p.m. (JST) time slot in nearly eleven years.
In addition to co-hosting Utaban with Tunnels member Takaaki Ishibashi, he serves as a co-host on The! Sekai Gyōten News(ザ・世界仰天ニュース,The World's Surprising News?), in which he shows a display of black comedy. As the leader of SMAP, Nakai has the greatest TV exposure of the SMAP members insofar as variety shows go.
In 2003, partly due to his involvement in very popular variety shows, Nakai was identified as the highest tax payer of any entertainer in Japan by the Japanese National Tax Bureau.