Ashino Masayasu served as a bushō during the Azuchi-Momoyama period and hatamoto during the early Edo period.  Masayasu was the seventeenth head of the Ashino clan based in Ashino in the Nasu District of Shimotsuke Province.

The Ashino were an illegitimate branch of the Nasu clan of Shimotsuke and counted among the Seven Clans of the Nasu.

Masayasu was born as the eldest son of Ashino Moriyasu, the sixteenth head of the Ashino clan with a fief of 800 koku.

In 1599, owing to the sudden death of Moriyasu, Masayasu succeeded him as head of the clan when he was only eight years old.  This same year, Masayasu traveled to Kyōto to meet Toyotomi Hideyori.  In 1600, he went to Edo and met Tokugawa Ieyasu for the first time.  For the ensuing Battle of Sekigahara, all of the members of the Nasu clan in the area sided with the Eastern Army led by Tokugawa Ieyasu.  Masayasu sent his elderly mother to Edo as a hostage to evidence he was free of duplicity and reinforced the defenses of the Ashino to prepare for a southern advance by Uesugi Kagakatsu.  Based on these contributions, after the war, his fief was increased by 300 koku.

In the Battle of Sekigahara, Satake Yoshinobu of Hitachi Province aligned with the Western Army.  After their defeat, in 1602, when Yoshinobu’s landholdings were transferred, Masayasu, along with Iōno Sukenobu, headed toward the village of Sannō in the territory of Yoshinobu to defend the area.  This year, his fief was increased by 1,600 koku.  As a result, the fief held by the Ashino clan totaled 2,700 koku.

Masayasu died in 1611 at the age of twenty.  He was succeeded by his lineal heir, Ashino Sukeyasu (Minbu-Shōyū).