[3][4] A televised performance of Generations from Exile Tribe turned Mamehara into their big fan and motivated him to begin attending a dancing school in his second year of junior high.

[8][9][10] Following an "intense" boot camp in South Korea,[6] seventeen-year-old Mamehara debuted with the resulting boy group JO1 and its first single, Protostar, on March 4, 2020, through Lapone Entertainment.

[14][15] He reprised his role in Birth of Chimera (2022), an exclusive Toei Tokusatsu Fan Club spin-off film that delves into his character's backstory.

[17][18] In the following year, he made his TBS Sunday Theatre debut in the Ryunosuke Kamiki-led drama Diamond Sleeping in the Sea [ja].

[19] In January 2025, Issei was cast in the lead role of Tsukasa Kiriki for the live action movie adaptation of the manga Bad Boys.