Before the Boshin War, Itoh had already relocated to Edo and had placed his naval skills at the service of the forces striving to overthrow the Tokugawa Shogunate.

He escaped from the burning of the Satsuma Domain residence in Edo and subsequently fought in many of the naval engagements of the Boshin War.

Promoted to captain in 1882, he served on many warships of the Imperial Japanese Navy in its pioneer days, notably the Ryūjō, Fusō, and Hiei.

On 15 June 1886, he was promoted to rear admiral, and made commandant of the Imperial Japanese Navy Academy.

Itō professed to have absolutely no interest in politics whatsoever, but was unofficially regarded as one of the genrō by his contemporaries.