Tosa Domain
Sasaki was born into a samurai class family in Agawa District, Tosa Domain (in the present-day city of Kōchi, Kōchi Prefecture). He served the Yamauchi clan in several important posts, including "kōri-bugyō" (country magistrate) and "ōmetsuke" (inspector). He also supported Sakamoto Ryōma in the "Taisei hōkan" movement to restore political power to the Emperor of Japan at the expense of the Tokugawa Shogunate. During the Boshin War, he commanded the "Kaientai", a paramilitary group of Tosa samurai youths, and occupied the Tokugawa magistrate's office at Nagasaki.