Manthatis began his career at Olympiacos and broke into the first team in the 2016/17 season as the Reds won the Super League 1 championship and reached the Round of 16 of the UEFA Europa League, making 18 domestic appearances and nine European appearances. He would go out on a series of loans to top flight clubs in Greece and Cyprus before eventually leaving Olympiacos in 2020.
Manthatis joined AKFC in January 2022. Playing primarily as a right-back, he quickly established himself as a key figure in the team that finished in second place in the 2021/22 season. Manthatis spent the second half of the 2022/23 season on loan to Finnish top flight club KTP, and returned for the 2023/24 season.
A former international, Manthatis represented Greece from the Under-17 to Under-21 levels.