Mutō was born in an ex"-samurai" family from Saga Domain. After graduating from the Imperial Japanese Army Academy, he served in the First Sino-Japanese War as a lieutenant in the infantry. After the war (and his promotion to captain) he was sent twice to Russia as a military attaché, spending time in Vladivostok and in Odessa. He was fluent in the Russian language, which proved invaluable during the Russo-Japanese War. After his promotion to major, then colonel, he returned to Japan in a posting with the Imperial Guards.