He then joined the civil service, working at various courts and as city councillor at Leipzig, Hainichen, Zittau and Meerane.

[1] In 1919, he joined the German Democratic Party (DDP), which he represented first in the Weimar National Assembly and then from 1922 to 1932 in the Reichstag.

[1] Elected as Oberbürgermeister (mayor) of Dresden in 1931, Külz was removed from office by the Reichskommissar for Saxony in March 1933, after he had refused to hoist a flag with the Nazi swastika over city hall.

[1] On 17 March 1947, in a conference in Rothenburg ob der Tauber Külz and Theodor Heuss were elected co-chairmen of the planned Democratic Party of Germany (DPD), aimed at uniting liberals of both the Soviet and the Western occupation zones.

Arthur Lieutenant, the spokesman of the LDPD on the matter, declared that under those circumstances and considering reproaches laid against East German liberals, no further co-operation was possible.

In March 1948, Külz once again was the representative of the LDPD at the Deutscher Volkskongress, organized at the behest of the Soviet authorities and the SED.

[1] On the morning of 10 April 1948, Külz was found by his party deputy, Arthur Lieutenant, to have died in the night at his Berlin apartment, apparently from a heart attack.