As the eldest brother, Fusamaru, died young, Tadakuni became an heir of the Karatsu Domain in 1805 and had an audience with the 11th Seii Taishogun (commander-in-chief of the expeditionary force against the barbarians), Ienari TOKUGAWA, and his heir, Ieyoshi TOKUGAWA in 1807.
長兄の芳丸が早世したため、文化(元号)2年（1805年）に唐津藩の世子となり、文化4年（1807年）に第11代征夷大将軍徳川家斉と世子・徳川家慶に謁見する。