He was the Deputy President of the Free City of Danzig (Gdańsk, Poland) from 1933 to 1934 in the Rauschning Senate, and was made Senate President (Senatspräsident) in 1935–1939. As Senate President of Danzig, he was a rival to his nominal superior Albert Forster, "Gauleiter" of the city since 1930. Greiser was part of the SS empire whilst Forster was closely aligned with the Nazi Party Mandarins Rudolf Hess and later Martin Bormann. On 23 August 1939 Forster replaced Greiser as Danzig's head of state.