Upon the Final Act of the 1815 Congress of Vienna, he was sent to the Greater-Poland capital Poznań as Duke-Governor and representative of Prussian King Frederick William III in the Grand Duchy of Posen. Struggling between his Polish subjects and the Prussian authorities, Radziwiłł found himself with little power, as effective power was executed by "Oberpräsident" Joseph Zerboni di Sposetti and the district governors heading the "Regierungsbezirke" of Posen and Bromberg. He unsuccessfully tried to oppose the Germanisation campaign begun by the Prussians. His daughter Elisa's engagement to Prussian Prince (later German Emperor) William I was broken in 1824.