Yokoyama Yui is a member of AKB48's Team A and the general manager of AKB48 groups.
Yokoyama's first attempt to join AKB48 or a sister group was for SKE48's second generation, but her audition was unsuccessful. She later passed the auditions with AKB48, and became a ninth-generation kenkyusei (trainee) member on September 20, 2009. She was the first of her generation to be promoted, which took place at AKB48 Tokyo Matsuri on October 10, 2010, and where she became a member of Team K.
In 2011, she secured her first participation on an AKB48 single title track by placing 19th in the AKB48 general elections. Her first A-side single was "Flying Get".
In 2012, she placed 15th in the AKB48 general elections.
In 2013, she placed 13th in the AKB48 general election and third in its rock-paper-scissors tournament.
In the AKB48 Tokyo Dome Concert, it was announced that Yokoyama would join NMB48 and therefore hold a concurrent position between AKB48's Team A and NMB48. In November, she sang on the A-side for NMB48's sixth single "Kitagawa Kenji". On April 28, 2013, NMB48 revoked her concurrency and she remained in AKB48.