A scholar, he turned his attention in 1906 to the completion of the Dai Nihonshi, a comprehensive history of Japan begun by his ancestor, Tokugawa Mitsukuni in the 17th century.

From December 1911, Kuniyuki Tokugawa served as a member of the House of Peers of the Diet of Japan.

On the completion of the Dai Nihonshi in 1929, he was awarded the title of koshaku (公爵, prince).

On June 25, 1940, he accepted the post of honorary president of Japanese Red Cross Society.

From October 11, 1944, to June 19, 1946, he served as the President of the House of Peers.