Tanaka auditioned for a spot in Morning Musume 5th Generation in 2001 and passed but had to leave the training camp when it was found out that she was underage. She attempted again in 2002 -- now old enough to audition -- for a position in the 6th Generation and passed along with other members Kamei Eri and Michishige Sayumi. Her audition song was Morning Musume's "Do it! Now". Once the 6th Generation joined, it made it the group's biggest formation yet, with 15 members in total.
In 2003, Tanaka was chosen to become the leader of a new Hello! Project unit alongside Hello! Project Kids members Suzuki Airi and Natsuyaki Miyabi. The unit was named Aa! and the group released their first and last single in October of the same year. The group later disbanded due to protests about the girls being too young to understand what they were singing about. However, Aa! still occasionally reunites to perform live.
She then provided the main vocals for the group's nineteenth single Shabondama, making her one of the few members of Morning Musume who was able to lead on her debut single. She was featured in two solo versions of Memory Seishun no Hikari on a fan club release CD. She released her first album with Morning Musume, Ai no Dai 6 Kan in December 2004.
During her first year in the group, Morning Musume was split into two groups: Morning Musume Otome Gumi and Morning Musume Sakura Gumi. Tanaka was put in Otome Gumi and released two singles with the group before they went on hiatus. 6th Generation members Fujimoto Miki and Michishige Sayumi also joined Otome Gumi, while Kamei Eri joined Sakura Gumi.
Tanaka was also one of the few members to be selected for the 2005 summer shuffle groups. She, along with fellow Morning Musume member Takahashi Ai, was a part of Elegies. They contributed the song Inshouha no Renoir You ni to the summer shuffle CD that year.
On March 28, 2008, it was announced that Tanaka would be voice acting as Kirara, a main character in Sanrio's anime Onegai My Melody Kirara, the fourth My Melody series. The show premiered on April 6, 2008.
In 2008, Tanaka became a member and a leader of Hello Project's new unit High-King, formed to promote their new musical; Cinderella the Muscial. The group released one single, C\C (Cinderella/Complex), and haven't released anything since.
In 2010, it was revealed that Tanaka would be voicing the main/title character of the new anime series, Kaito Reinya. The character, Reinya, had initially been modeled on Tanaka. The show started airing on Januaury 9th, 2010 and is set to have a total of 12 episodes.
In 2010, Tanaka opened her first blog with the title Tanaka Reina no Otsukareina.
Tanaka, along with 6th generation member Michishige Sayumi, hosted a premium Christmas dinner show. The date was 12/23 and it was hosted at the Prince Hotels.
On January 02, 2012 it was announced that Niigaki Risa will graduate from Morning Musume and Hello! Project on May 18, 2012. Presumably, Morning Musume's leadership will be passed on to Michishige Sayumi and sub-leader will be Tanaka.
On April 18, it was announced that Tanaka and Morning Musume's 9th & 10th Generation members will star in a new stage play titled Stacey’s Shoujo Saisatsu Kageki. The musical will run from June 6-12.
It was announced Tanaka will be the center in a new Hello! Project group with new vocalists and a guitarists. The audition for the new vocalists and a guitarists will start on June 17 and continue for 4 weekends consecutively.