Yamauchi Toyonori was the 14th son of Yamauchi Toyoshige, 12th daimyō of Tosa Domain. In 1848, his elder brothers Yamauchi Toyoteru (13th daimyō) and Yamauchi Toyoatsu (14th daimyō) died of illness within a few months of one another. At the time, Toyonori was only age three, so the clan elders decided to make his elder cousin, Yamauchi Toyoshige the 15th daimyō. Yamauchi Toyoshige (also known as Yamauchi Yōdō) was forced into retirement in 1859 due to the Ansei Purge undertaken by the tairōIi Naosuke, making Toyonori the 16th daimyō. Yamauchi Yōdō continued to wield political power behind-the-scenes, especially after his ban was officially lifted in 1862, and Toyonori's hope of activity was severely limited. He was sent to Kyoto to guard the Imperial Palace in 1862. In 1869, together with representatives from Satsuma Domain and Chōshū Domain, he officially returned the domain to the control of Emperor Meiji. During the Boshin War, he sought a pardon for his father-in-law Uesugi Narinori of Yonezawa Domain.