[2] Sontheimer played with the Fürth youth teams as a regular starter in the Under 17 Bundesliga and Under 19 Bundesliga, before he made his first brief appearance in the senior division in October 2016 at the age of 18 when he played for the Greuther Fürth II team in the Regionalliga against FC Augsburg II.
In the winter training camp of the 2016–17 season, Sontheimer was temporarily allowed to practice with the first-team squad under head coach János Radoki, who knew him from the U19s.
[2] In the first match of the new year against 1860 Munich, Sontheimer was in the starting line-up and made another ten appearances during the spring.
[5] At the end of February 2020, the Kickers triggered the option to buy and Sontheimer, who had become a regular starter for the team, stayed in Würzburg permanently.
He had only missed one league game due to a yellow card suspension and was named the second best defensive midfielder in the 3.