Ai Takahashi joined Morning Musume in 2001 as part of the group's fifth generation, along with Makoto Ogawa, Asami Konno, and Risa Niigaki.
Her audition involved a training camp of three days and two nights, in which participants were expected to learn a new song, a dance routine and a script. Out of 25,000 applicants, nine were selected for the camp in which Takahashi passed.
Her debut with the group was on their single "Mr. Moonlight: Ai no Big Band", and her first appearance on a Morning Musume full-length release was their fourth studio album, 4th Ikimasshoi!. Her first appearance within a Hello! Project shuffle unit release was on the 2002 single "Shiawase Beam! Suki Suki Beam!" under the group name Happy 7.
In 2003, Takahashi replaced Mari Yaguchi in the Morning Musume spinoff group Mini Moni, first appearing in the group's movie Mini Moni ja Movie: Okashi na Daibōken! and its accompanying soundtrack. Her best-known nickname amongst non-Japanese fans, "Takitty", was derived from the cat suit (complete with tail) which Ai Takahashi wears in the movie.
Later in 2003, Ai Takahashi was part of the Morning Musume splinter group Morning Musume Sakuragumi, which performed mainly slower numbers on the group's two EPs, "Hare Ame Nochi Suki" and "Sakura Mankai", and the shuffle group 7Air, an R&B-inspired septet.
Takahashi's vocals became more prominent on the second and final Mini Moni album, Mini Moni Songs 2 (2004), as well as on Morning Musume's singles from their Spring 2004 release "Roman: My Dear Boy" onward. Ai Takahashi also had a duet with Tsunku on the cover version of Tsunku and Ayumi Hamasaki's duet "Love: Since 1999" on his solo album Take1.
In 2005, Morning Musume's first single release of the year, "The Manpower!!!", featured Takahashi in a prominent co-lead-vocal role, a role Ai Takahashi has retained on subsequent singles since. In the summer of that year, she became part of the 2005 shuffle Group Elegies.
In 2006, Takahashi played the lead role, Sapphire, in the musical "Ribbon no Kishi The Musical" which was a collaboration work of Takarazuka Revue (a famous Japanese all-female musical group) and Hello! Project. The musical was based on Tezuka Osamu's manga and also starred v-u-den, Nozomi Tsuji, Aya Matsuura, and Natsumi Abe, as well as Marcia and Kaoru Ebira of the Takarazuka Revue. Soon thereafter, Takahashi released her first and only solo single to date, "Yume Kara Samete".
Following then-leader Hitomi Yoshizawa's graduation from Morning Musume on May 6, 2007, the current sub-leader, Miki Fujimoto ascended to the leader position, with Takahashi becoming the new sub-leader of the group. However, on June 1, 2007, after Fujimoto's resignation, Takahashi took over as the new leader.
Takahashi is also captain of the Hello! Project kickball team, Metro Rabbits H.P..
In 2008 Takahashi became a member of Hello Project's new unit High-King, a group created to promote Morning Musume's upcoming Cinderella the Musical, in which Takahashi will play the main character.
Takahashi is a fan of Broadway and musicals such as Phantom of the Opera and Chicago. Ai Takahashi harbors an ambition to one day perform in a New York production. Ai Takahashi is also a fan of Destiny's Child.
It was announced in July 2008, that Takahashi and fellow Morning Musume member Risa Niigaki would play the 80s J-Pop duo Pink Lady in the TV Drama Hitmaker Aku Yuu Monogatari.
As of January 2009, Ai Takahashi and Risa Niigaki have the longest tenures of any member. Ai Takahashi is also one of only seven members to remain in the group for seven years or more (the others being Kaori Iida, Yoshizawa, Niigaki, Eri Kamei, Sayumi Michishige and Reina Tanaka) and one of two (along with Niigaki) to remain in the group for eight or nine years.
On February 1, 2009, during the "Hello Pro Award '09 ~Elder Club Sotsugyō Kinen Special~" concert held at Yokohama Arena, Yuko Nakazawa passed on her leadership position in Hello! Project to Takahashi.
Around December 2009, Takahashi was one of many people to advertise for the release of Final Fantasy XIII in Japan.
In September 2010, Ai Takahashi became the second Morning Musume member to be at least 24 years old and the first to reach that age while in the group (original member Yuko Nakazawa was 24 when the group was formed).
On January 9, 2011 it was announced that Takahashi would graduate from Morning Musume at the end of the Fall tour that year, passing leadership of Morning Musume and Hello! Project to current sub-leader Risa Niigaki.