As capital of the Kingdom of Prussia, Berlin received its first Lord Mayor (Oberbürgermeister) according to the Prussian reforms after the retreat of the Napoleonic occupation troops in 1809, approved by King Frederick William III. He served as head of the city council called Magistrat. The two-stage administration and the office of the boroughs' mayors were implemented in the course of the wide-ranging incorporations by the 1920 Greater Berlin Act.