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.

[3] 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.

[5] On 10 October 2023, Mostovoy extended his contract with Zenit once more until June 2027 with an option for the 2027–28 season.

[10] On 11 June 2021, he was removed from the squad after testing positive for COVID-19 and replaced by Roman Yevgenyev.

[11] He scored his first international goal on 11 November 2021 in a World Cup qualifier against Cyprus.