Akechi Mitsuyasu served as a bushō and was the third son of Akechi Mitsutsugu, lord of Akechi Castle (also known as Nagayama Castle) in Mino Province.

In 1535, Mitsuyasu’s older brother, Mitsutsuna, successor to Mitsutsugu as head of the clan and lord of Akechi Castle, died young.  His son, Mitsuhide, was very young so Mitsutsugu was ordered out of retirement to serve as guardian for Mitsuhide.  After Mitsuhide’s coming-of-age ceremony, Mitsutsugu continued to lead the clan.   Mitsuhide may have refused the role as head of the clan.  In 1547, Mitsuyasu visited Ashikaga Yoshiteru, the shōgun, and received the Court titles of Junior Fifth Rank – Lower Grade, and Director of Military Storehouses.  In 1553, he entered a Buddhist monastery and assumed the name of Sōshuku.

After Saitō Dōsan usurped the Toki and became shugo of Mino, Mitsuyasu established relations with Dōsan, and offered his younger sister, Omi-no-kata, as a second wife.  The conflict between Dōsan and his son, Tatsuoki, deepened, and in 1556, Dōsan and senior commanders including Inaba Yoshimichi were killed in battle against Yoshitatsu.  This placed Mitsuyasu, as a maternal relative of Dōsan, in opposition to Yoshitatsu, whereupon Mitsuyasu, his younger brother (Mitsuhisa), and commanders from the Miyake, the Fujita, and the Tsumaki holed up in Akechi Castle.  Supporters of Yoshitatsu including Ibi Mitsunari and Nagai Michitoshi attacked the castle.  Defensive measures failed, leading Mitsuyasu and Mitsuhisa to kill themselves.  Mitsuyasu entrusted his son, Akechi Hidemitsu, to Mitsuhide, as designated heir to the clan, and enabled his escape from the castle.