Jan Fortelný has grown in Spartan academy. In the 2016/17 and 2017/18 seasons, he played 20 league games for the U19 team and added 7 UEFA Youth League matches. Last season he was on loan in Vlašim. He played 21 matches and scored two goals. During the 2019/2020 season, he was on loan in Jihlava. During the 2020/2021 and 2021/2022 season, he was on loan in Teplice.