Hildebrandt Diehl is a former mayor of Wiesbaden. From 1989 to 1997, Diehl served as mayor of the Hessian state capital.

In the 1997 mayoral election, he managed to win a narrow majority in the run-off election against his opponent, who was still ahead in the first round. In the subsequent elections in 2003, Diehl received 57.7% of the vote in the first round and was thus confirmed in office.

Having reached the age limit, Diehl retired from office on July 2, 2007 before the end of his term of office.

Hildebrand Diehl has been an honorary citizen of Wiesbaden's twin city Görlitz since 2000. On the occasion of his retirement from office, he signed the city's Golden Book on June 30, 2007.