Schmitz started playing football at local Munich club SV Waldperlach. In 2001, he joined the youth ranks of Bayern Munich where he eventually was promoted to their reserve team in 2012. Having failed to establish himself at the first team, he left the club and signed for Austrian champion Red Bull Salzburg in the summer of 2014.[3]
On 23 August 2014, he made his league debut in a 5–0 home win against Rheindorf Altach by coming on as a substitute for Christian Schwegler after 83 minutes.[4] In 2018, after a quiet stint with Leipzig, Schmitz was transferred to newly relegated side 1. FC Köln for a fee of €1.5 million.[5] In March 2022, having established himself in the club's starting line-up during the 2021–22 season, Schmitz extended his contract at Köln until June 2024.[6]