Kojima started her career as a member of Angel Eyes, an idol group under Stardust Promotion that disbanded in 2001. Initially, she was under contract with Stardust Promotion when she was still with Angel Eyes, but later switched to Ogi Production and stayed with the agency ever since.
Kojima participated in the first AKB48 audition in July 2005, which she passed along with 23 other participants, because the application was just taking a self image by a mobile phone and e-mail it, and assuming that she "could take 10-time prettier one, and just sent". Although she voluntarily applied for the audition and passed, she was still looking at then-AKB48 project in a suspicious way, and tried to skip the very first dancing lesson because of her then-part time job, but eventually participated it in response to the persuasion by Tomonobu Togasaki, Manager of AKB48 Theater. The group made their debut in December of the same year, and Kojima was assigned as one of the 20 members of Team A; the number later dropped down to 16 but Kojima remained as a member. In the past few years, she had made several appearances on television with the group, and had participated on the A-sides in all of the group's singles along with her teammate Atsuko Maeda, making her one of the most visible figures in the group. Kojima made her first appearance on Kohaku Uta Gassen, an annual music show where the most successful artists of the year are invited, with AKB48 on December 31, 2007.
Since 2007, Kojima has been actively exploring a possible acting career, and has made several appearances in a number of dramas and movies. Her first lead role came in the drama Coin Locker Monogatari (コインロッカー物語?, literally Coin Locker Story), which was filmed and aired in early 2008. Additionally, Kojima made her first appearance on the silver screen in the horror movie Densen Uta (伝染歌?), along with several fellow AKB48 members. The group has also made brief appearance at the beginning of the movie in a live performance.
She dropped out of high school at the age of 16 due to school regulation which does not allow students to work in the entertainment industry.