Born in 1823 in Honjo, Edo, to a low-ranking bannerman family. He studied Dutch learning and military science through self-education.
In 1860, he captained the Kanrin Maru across the Pacific — the first transoceanic voyage by Japanese sailors. This experience broadened his worldview and formed the basis of his advocacy for opening Japan.
In 1864, he established the Kobe Naval Training Center where he mentored Sakamoto Ryoma and other future leaders, transcending class and domain barriers.
In March 1868, entrusted with full authority by Yoshinobu after Toba-Fushimi, Kaishu negotiated directly with Saigo Takamori at the Satsuma residence in Takanawa. Over two days of talks, he achieved the bloodless surrender of Edo Castle, saving over a million lives. This is considered the greatest diplomatic achievement in Japanese history.
After the Meiji Restoration, he served in various government positions while working to support former Tokugawa retainers. He died on January 19, 1899, at age 77, and was buried by Senzoku Pond — where a memorial to his former adversary Saigo Takamori stands nearby.