Hagiwara Mai took part in the 2002 auditions at the age of 8 for Hello! Project Kids and passed along with fourteen other girls. Her audtion song was "Te wo Nigitte Arukitai" by Goto Maki. She made her first appearance in the entertainment industry as a member of 4KIDS, a temporary group that appeared in Minimoni's movie Minimoni ja Movie Okashi na Daibouken!.
Hagiwara continued her work within the Hello! Project Kids, namely performing alongside Morning Musume at concerts as backing dancers. In late 2004 Hagiwara took part in her first Hello! Project shuffle unit, H.P. All Stars, taking part alongside all the members of Hello! Project at the time. The shuffle unit released one single and became inactive by January 2005.
In 2005 Hagiwara was assigned as a member of Hello! Project group °C-ute alongside seven other Hello! Project Kids members. These eight members had been passed over when it came to the forming of Berryz Koubou, which had originally been formed to teach the Hello! Project Kids about being in a group by rotating the line-up amongst the kids. Originally °C-ute was releasing on an indie label, but in 2007 they made their major debut.
Hagiwara Mai released four indie singles as a part of °C-ute in this year as a preparation for the major debut in 2007. °C-ute also released their first album, "Cutie Queen Vol. 1".
On February 21, 2007 °C-ute released their debut single, "Sakura Chirari". On the first day in the charts it ranked third on the Oricon charts, a feat not even accomplished by Hello! Project's mother group Morning Musume (who had ranked 6th on their debut single). With their debut single, they became the youngest group (with the average age of 13) to rank in the top 10. Hagiwara provided the minor vocals for their debut single.
Hagiwara also joined with Morning Musume's Kusumi Koharu to form a oneshot unit named Kira☆Pika. The group was formed to perform songs for the anime Kirarin Revolution, as both girls lend their voices to characters in the series. They released their debut single on August 1, 2007.
Hagiwara continued her duties within °C-ute, releasing four singles and a third studio album. Hagiwara provided the minor vocals on °C-ute's fourth major single, "LALALA Shiawase no Uta ".
°C-ute released their fourth studio album, "4 Akogare My Star", on January 28th and their 8th major single is scheduled to be released on April 15th, titled "Bye Bye Bye!".
Hagiwara Mai, along with four other members of °C-ute and Berryz Koubou left the Hello! Project futsal team, Gatas Brilhantes H.P.. The members left of their own violition due to feeling that they could not perform as high level players due to their level of ability.
On December 15, Hagiwara Mai opened her first Ustream channel called 'Mai Desu ga... Nani ka?' In the show, Hagiwara invites various Hello! Project members to do activity with her such as talking and cooking.