Mittelstädt made his first footballing steps as a five-year-old in the youth team of SC Staaken before moving to Hertha 03 Zehlendorf.

[3] Mittelstädt made his professional league debut on 2 March 2016 against Eintracht Frankfurt,[4] coming as a substitute in the 90th minute for Salomon Kalou.

[5] On 30 October 2018, he scored his first competitive goal for Hertha BSC in a 2–0 away victory in the DFB-Pokal against SV Darmstadt 98.

[7] In the 2023–24 season, he achieved the highest dribbling success rate in the Bundesliga at 61%, playing a crucial role in his club's qualification for the UEFA Champions League.

[10] In March 2024, Julian Nagelsmann called him up for the German national team ahead of the friendly matches against France and the Netherlands.