Mostovoy made his debut in the
Russian Professional Football League for
FSK Dolgoprudny on 10 April 2016 in a game against
FC Znamya Truda Orekhovo-Zuyevo and scored a goal on his debut in his team's 2–0 victory. On 19 July 2019, he joined
PFC Sochi on loan for the 2019–20 season. Upon his return from loan, on 7 August 2020 he made his debut for
FC Zenit Saint Petersburg's senior squad, as a substitute in the
2020 Russian Super Cup game, which Zenit won. On 5 May 2021, he extended his contract with Zenit throughout the 2023–24 season. On 10 October 2023, Mostovoy extended his contract with Zenit once more until June 2027 with an option for the 2027–28 season. ==International career==