Sumiaki was sickly, and retired from his duties in December 1846, and Sumihiro became the 12th daimyo of Ōmura on February 21, 1847.

Sumihiro was an active ruler, interested in both rangaku and classical learning, and concerned with the direction the country was taking into the unsettled Bakumatsu period.

Sumihiro was considered a strong Tokugawa loyalist, and in 1863 was entrusted with the important position of Nagasaki bugyō, but defected after only a year to become a supporter of the Sonnō jōi movement.

During the Boshin War of the Meiji Restoration, he commanded his forces as part of the Satchō Alliance in support of Emperor Meiji, and fought against the Tokugawa remnants of the Ōuetsu Reppan Dōmei in northern Japan.

In June 1868, in return for his loyalty to the new government, the revenues of Ōmura domain were raised to 30,000 koku.