Accompanied by novelist Ivan Goncharov and inventor Alexander Mozhaisky, Posyet explored and mapped the northern coastline of the Sea of Japan, including Possiet Bay, which now bears his name.

Back in the capital by 1858, Posyet was asked to supervise the education of Grand Duke Alexei Alexandrovich of Russia, who was destined for a naval career.

In 1871-72 they visited the United States, the imperial party comprising, besides Posyet and Grand Duke Alexei, Councilor Machin, Consul Bodisco (who helped negotiate the sale of Alaska in 1867), Count Olsonviev, Dr. Kudrin, and Lieutenants Sterlegov and Sten Carl Tudeer (later governor of Vyborg and a member of the Finnish Senate).

Posyet and his pupil inspected the condition of U.S. railroads and inner waterways and met President Ulysses S. Grant, Hamilton Fish and other leading politicians.

Inspired by his American experience, he set himself the task of extending Russia's network of state-owned railways and upgrading the obsolete Mariinsky Canal System.