He was drafted as a member and co-lead vocalist of the popular J-pop group KAT-TUN in 2001. The group officially debuted in 2006, and has since achieved a string of chart successes with all six albums and 21 singles reaching number 1 on the Oricon charts. He is also one half of the temporary group, Shūji to Akira, whose only single "Seishun Amigo" became the best-selling single of 2005 in Japan.
Born in Edogawa, Tokyo on February 23, 1986, Kamenashi is the third child of his parents. He has two older brothers, Yūichirō and Kōji, and one younger brother, Yūya. He also has two sisters-in-law and a niece from his second brother's marriage and a nephew from his first brother's marriage. His given name comes from a character from the manga, Touch, written by a Japanese manga artist, Mitsuru Adachi. Coincidentally, bandmate Tatsuya Ueda was also named after the twin brother of Kamenashi's namesake.
An avid baseball fan and player like the character he was named after, Kamenashi once represented his country in the junior world league as shortstop though he had to give up the sport professionally due to lack of spare time after he was accepted into Johnny's Entertainment despite having the support of Johnny Kitagawa, the agency's president, to pursue both careers.
In 2009, Kamenashi was cast in a live-action adaptation of a wine-themed manga, Kami no Shizuku, as the leading actor. Voters awarded him, his co-stars (Riisa Naka and Seiichi Tanabe) and the drama a near clean sweep of the winter edition of the Nikkan Sports Drama Grand Prix Awards in March 2009. He made a guest appearance as a doctor on the third episode of TBS drama series, Mr. Brain, opposite his talent agency senior Takuya Kimura. Kamenashi also made his film debut on July 11, 2009, reprising his role of Ryū Odagiri, now a trainee teacher in Gokusen The Movie. The movie earned half a billion yen in its opening weekend landing at the top spot of the box office and stayed in the top ten for six consecutive weeks. It later ranked at number 16 on Japan's 2009 yearly box office results raking in 33,963,369 USD at the end of its run.
In 2010, he was cast in the leading role of Kyohei Takano for a live-action adaptation of popular manga, Yamato Nadeshiko Shichi Henge, which aired on NTV. It was named Best Drama by voters of the 13th Nikkan Sports Drama Grand Prix Awards in March 2010 while Kamenashi and his co-star, Aya Omasa, were also awarded Best Actor and Best Supporting Actress respectively.
In 2011, he took on a role as Bem from the adaptation of anime Yōkai Ningen Bem. He received Best Actor award from Nikkan Sports Drama Grand Prix and Television Drama Academy Awards that year. Following the success of the drama, Yōkai Ningen Bem was made into film in 2012. At the end of its run, the film had grossed US$12,628,578 gross.
In 2013, Kamenashi starred in the movie It's Me, It's Me. The movie premiered in 15th Udine Far East Film Festival Italy in April, and Kamenashi won the My Movie Audience Award.
Kamenashi walked the runway in 2008 for Hermès' men's autumn/winter 2008 collection in Tokyo. In 2010, Kamenashi won the annual Best Jeanist award with more than 29,000 votes in the men's category, almost triple that of second runner-up, Masaki Aiba. This is his fifth consecutive win and is thus promoted into the hall of fame, making him Eternal Best Jeanist and ineligible for future runs.
Kamenashi participated in the Central League's Yomiuri Giants 23rd FanFiesta 2009 on November 23, 2009 and led his team to victory. He participated during the 1st inning as shortstop and for the 2nd and 3rd innings as pitcher. He belonged to the Black Rose Variety Team led by SMAP's Nakai Masahiro against the Giants Select Members Team.