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 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, the appropriately named 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 she wears in the movie.
Later in 2003, she 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. She 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 she 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.
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, she and Risa Niigaki have the longest tenures of any member. She is also one of only four members to remain in the group for seven years or more (the others being Kaori Iida, Yoshizawa and Niigaki).
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.