In the annual AKB48 General Elections event, she had consistently been voted by fans to rank among the group's top members.

[1] Watanabe auditioned for AKB48's 3rd generation and was placed in the newly formed Team B, in which she had been the face of since formation.

She would go on to appear on all of the a-side tracks for AKB48's singles until her graduation, with the exception of line-ups determined by rock-paper-scissors tournaments.

[8] In 2012, Watanabe got her first lead role in the Japanese TV drama Saba Doru, where she played a 38-year-old high school teacher hated by her pupils, whilst having a hidden life as the highly popular idol Mayu Watanabe, based on her real-life self.

On February 29, Watanabe released her debut solo single, "Synchro Tokimeki", which is also the opening theme for the drama.

[10] In the anime, she plays a main character Chieri Sono,[11] and is part of the AKB48 subgroup NO NAME, which sings the opening and ending themes for the series.

The series features a supporting character, Mayu Watanabe Mark 3, which is modeled after her likeness, and is voiced by Yukari Tamura.

[14] On July 25, she released her second solo single, "Otona Jellybeans"[15] which reached number three on the Oricon weekly chart.

Its b-side "Sayonara no Hashi" is the theme song to the anime film Nerawareta Gakuen for which she voiced one of the lead characters.

[21] The theme song for this drama, "Deai no Tsuzuki", was sung by Watanabe, and was also released as her fifth solo single on June 10, 2015.

[29] Her results in the annual ranking:[30] In January 2018, it was announced that Watanabe would play the titular character in the Japanese production of the Amélie adapted from the movie of the same title.

[31] In April 2018, Watanabe had her first drama post-AKB48 titled Gan Shoumetsu no Wana ~Kanzen Kankai no Nazo~ (がん消滅の罠 ～完全寛解の謎～) where she played an insurance investigator.

The commercial also features some 33 professional dancers and performers including Spinboy Aichi, SORI and HICKY of the dance team SHIFFLE!!

[39] Tokyo Mode describes Watanabe as a girl who used to be an "otaku obsessed with illustrations, anime, and special effects shows".

[40] Her public appearances were often accompanied by life-sized animated characters and many fans wore such costumes at her meet-and-greet events.

[41][42] In several episodes of the AKB48 variety show AKBingo!, she demonstrated her specialty of drawing various manga characters in seconds.

In the early years of her career she was called "CG" or "cyborg" due to her classic, orthodox idol appearance.