Since joining Johnnys, he has been a member of Jr. groups B.I.G., Four Tops, and most recently NEWS. Yamashita began to appear in television dramas soon after he joined Johnnys. In 2000, at age 15, his performance in Ikebukuro West Gate Park earned him an award for "Best New Comer". That same year, he was voted first place in the Annual Junior Awards for "Best Boyfriend", "Most Beautiful", to name a few for the first time.
When Takizawa made his debut with Tsubasa Imai as Tackey & Tsubasa in 2002, he left as the leader of Johnny's Juniors. Around the same time, Yamashita and the three most popular Tokyo Juniors Toma Ikuta, Jun Hasegawa, and Shunsuke Kazama formed 4Tops. 4Tops then began to host the Junior television program, The Shōnen Club.
In September 2003, eight other boys from Johnny's Juniors were chosen to form the group NEWS with Yamashita as the leader. NEWS' formation came as a shock to not only the fans of Four Tops but its own members as well. Yamashita later revealed that at the time, aside from Ryo Nishikido, he had barely talked to any of the other group members. NEWS made its formal debut on May 12, 2004.
In October 2005, Yamashita and Kamenashi Kazuya from KAT-TUN formed the temporary unit, Shuuji to Akira, to promote their television show Nobuta wo Produce which also starred Horikita Maki. (Shuuji and Akira are the characters they portray in the drama.) They also released a single, Seishun Amigo, under the name Shuuji to Akira, which sold over a million copies in four weeks. Seishun Amigo topped the Oricon Yearly Single Rankings for 2005.
In spring 2006, Yamashita recorded "Kurosagi: The Black Swindler", a drama based on the manga of the same name. Kurosagi is basically about the art of swindling. For this purpose, Yamashita recorded his single "Daite Señorita" which was #1 in the Oricon ranking. Horikita Maki also starred with Yamashita in this drama.
In the latter half of 2006, Yamashita joined a Thai duo called Golf-Mike, who are brothers, to form a temporary unit called Kitty GYM along with Kitayama Hiromitsu (Kis-my-ft2), Inoo Kei (J.J. Express), Totsuka Shota (A.B.C.), Yaotome Hikaru (Ya-Ya-Yah). The unit was created to cheer on the Japan Women's Volleyball team and they released a single in August called "Fever and Future" under the name "GYM" which stands for Golf, Yamashita and Mike, respectively. Yamashita returned as a member of NEWS on December 31, 2006 on Johnny's Countdown 2006–2007.
Yamashita, along with KAT-TUN's Koki Tanaka and Kis-my-ft2's Taisuke Fujigaya, then starred in Byakkotai, a mini-drama in two parts that aired in January 2007. The drama special featured the story of Sakai Mineji one of the members of Byokkotai.
In April 2007, Yamashita was casted in a romance drama series called Proposal Daisakusen alongside Nagasawa Masami as Iwase Ken. This was his first lead role in a Fujitv primetime drama. The drama gained a strong following which resulted in the production of a special the following year.
In July 2008, Yamashita starred in the medical drama Code Blue, playing flight doctor in training Aizawa Kosaku, alongside actresses Aragaki Yui and Erika Toda. The Code Blue special was later broadcasted in January 2009. The success of the drama and special is evident as Yamashita and the main cast reprised their roles for Code Blue 2. The time slot of the series changed to Mondays at 9pm marking Yamashita's third time in the popular timeslot.
In the summer of 2009, Yamashita starred as the leading actor in a sports/romance drama called Buzzer Beat alongside Kitagawa Keiko who played an aspiring musician. On 30 August, NEWS hosted NTV's 24Hour Television event for charity. Following the release of his second solo single, Loveless, Yamashita held his first solo concert, Short but Sweet.
During the early months of 2010, Yamashita was busy filming for the upcoming 2011 movie, Ashita no Joe, which is a live-action adaptation of the manga of the same name. In July of 2010, Yamashita released his third solo single 'One in a Million' which went to number 1 on the Oricon charts.
During the latter half of 2010, Yamashita has been busy with NEWS activities. NEWS released a new album in September entitled 'LIVE' which contained 'Sakura Girl' as well as 'Koi no ABO'. 'LIVE' was NEWS' first album since 2008. Alongside the album, NEWS held their 'LIVE! LIVE! LIVE! NEWS DOME PARTY 2010'. During the Tokyo Dome concert it was announced that NEWS would be releasing their 13th single entitled 'Fighting Man'.
