Masaoki Shimmi was a Shogunate retainer who successively held the titles of Ise-no-kami and Buzenno-kami. He was appointed chief delegate in a Shogunate mission to the United States in 1860. Norimasa Muragaki and Tadayori Oguri were other high-ranking officials included in his party of 81 men.
The Japanese delegation left Uraga aboard the U.S. warship Powhatan. Masaoki presented to President Buchanan Shogun Iemochi's message and friendship treaty documents. After treaty documents were exchanged with Buchanan, Masaoki's party inspected various parts of the United States, crossed the Atlantic aboard the Niagara, the largest American warship of that period, and sailed back to Shinagawa, Edo via the Cape of Good Hope and Indian Ocean.
Naosuke Ii, the Shogunate Prime Minister, who had sent Masaoki to the United States, had been assassinated and Japan was agog with antiforeign ideas. Masaoki then was appointed sobashu (privy councilor) (1862) and accompanied Shogun Iemochi Tokugawa when he went to Kyoto to pay respects to the Court.