FC St. Pauli Flach made his professional debut for
FC St. Pauli in the
2. Bundesliga on September 27, 2020, coming on as a substitute in the 85th minute for
Jannes Wieckhoff against
1. FC Heidenheim, which finished as a 4–2 home win. Flach scored his first professional goal for FC St. Pauli on January 3, 2021, scoring in the 82nd minute against
Greuther Fürth in a 2–1 loss.
Philadelphia Union In March 2021, Flach signed a two-year contract with
Philadelphia Union competing in
Major League Soccer. Flach's debut with the Union was starting in the club's first
CONCACAF Champions League match, defeating
Deportivo Saprissa 1–0. Flach scored his first goal for the Union on October 3 against reigning MLS Champions,
Columbus Crew in a 3–0 win. His performances throughout the 2021 season made Flach a regular starter in the Union midfield and was recognized as being named among the MLS's 22 under 22.
Jagiellonia Białystok On January 15, 2025, free agent Flach joined Polish defending champions and
UEFA Conference League participants
Jagiellonia Białystok on a two-and-a-half-year deal, with an option for another year. == Personal life ==