Because of internal party controversies about the political course Ernst Reuter's time in the Communist Party's leadership was rather short, and already in January 1922 he was expelled from the party. Subsequently, he oriented himself anew to social democracy. After working as the editor for local politics for the SPD's publication Vorwärts (Forward), Ernst Reuter was elected Berlin city commissioner for transportation in October 1926. In this position he significantly contributed to the founding of the Berliner Verkehrs AG (BVG), Berlin Transport Corporation, which in 1928 was the largest local traffic enterprise in the world. It was here that Reuter could apply his previous ideas of local politics to city planning and traffic policy in a fast growing metropolis, where his responsibilities also encompassed the expansion of Berlin's city train and subway network.
A few years later he was offered a new challenge: From 1931 until 1933 Ernst Reuter served as mayor of Magdeburg where he attempted to draw on his experience which he had made in Berlin. A passionate defender of the Weimar Republic, he, however, was not able to meet up to the challenges of the Great Depression and the rise of the National Socialists.