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 appearance 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 on a Hello! Project shuffle unit release was on the 2002 single "Shiawase Beam! Suki Suki Beam!" under the group name Happy 7.
In 2003, she was tapped to replace Mari Yaguchi in the Morning Musume spinoff group Mini Moni, first appearing in the group's movie Mini Moni Ja Movie: Okashina Daibouken and its accompanying soundtrack. Her best-known nickname amongst non-Japanese fans, "Takitty", derived from the cat suit (complete with tail) that she wears in the movie.
That same year, 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, Minimoni 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.
Takahashi is also captain of the Hello! Project kickball team, Metro Rabbits H.P..
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 is based on Tezuka Osamu's manga and also stars v-u-den, Nozomi Tsuji, Aya Matsuura, Natsumi Abe, and Marcia and Kaoru Ebira of the Takarazuka Revue.
On January 2, 2007 at Hello! Project's winter concert, it was announced that Hitomi Yoshizawa, the leader of Morning Musume would graduate on May 6. After her graduation, Takahashi will become the new sub-leader of Morning Musume.
