The term Ōu (奥羽) is often used to refer to the combined area of Mutsu and the neighboring province Dewa, which together make up the entire Tōhoku region.

Mutsu, on northern Honshū, was one of the last provinces to be formed as land was taken from the indigenous Emishi, and became the largest as it expanded northward.

The ancient regional capital of the Kinai government was Tagajō in present-day Miyagi Prefecture.

In 1189, Minamoto no Yoritomo invaded Mutsu with three great forces, eventually killing Fujiwara no Yasuhira and acquiring the entire domain.

The fifth of these, corresponding roughly to today's Aomori Prefecture, was assigned the same two kanji as the entire province prior to division; however, the character reading was different.