Leon Maximilian Flach was born on 28 February 2001 in Humble, Texas, United States of America. He was born in Texas to German parents while his father was working in Houston before returning to Hamburg as a child, and he has represented Germany and the United States at youth level. Flach made his professional debut for FC St. Pauli in the 2. Bundesliga on 27 September 2020 in the match against 1. FC Heidenheim. He scored his first professional goal for FC St. Pauli on 3 January 3 2021 scoring in the 1-2 loss against SpVgg Greuther Fürth.