Ishikawa Masanobu served as a bushi during the Azuchi-Momoyama period.  He was a retainer of the Nanbu clan, a sengoku daimyō of Mutsu Province.

Masanobu was born as the son of Ishikawa Takanobu, a bushō and lord of Ishikawa Castle in Mutsu.  Nanbu Nobunao, the twenty-sixth head of the Nanbu clan, was Masanobu’s older brother.

In 1581, after the death of Takanobu from illness, Masanobu inherited the role as the district governor of Tsugaru.  Masanobu received support from Sentoku Masayoshi (the lord of Aseishi Castle), Daikōji Mitsuchika (the lord of Daikōji Castle), and Ōura Tamenobu (later known as Tsugaru Tamenobu) to serve as the central figure for the Nanbu clan governing the Tsugaru region from his base at Namioka Castle.  However, Sentoku Masayoshi soon died of illness, while Daikōji Mitsuchika was ousted after being slandered by Tamenobu.  In 1588, Masanobu was said to have been poisoned to death by Tamenobu.

Between 1571 and 1582, however, Ōura Tamenobu had already garnered control of the Tsugaru region, so Masanobu could not have entered Tsugaru at this time.  This raises questions among some scholars as to the existence of Masanobu.  Meanwhile, Nanbu Nobunao, a mainstay of the Nanbu clan, the son of Ishikawa Takanobu, and the designated heir of the Ishikawa family, was adopted to become the heir to Nanbu Harumasa of the main branch of the Nanbu clan.  Therefore, it is improbable that the Ishikawa family would have been abandoned with no designated heir.  In the records of the Tsugaru clan, in 1571, Ōura Tamenobu attacked Ishikawa Castle during which Masanobu’s father, Takanobu, was killed, while Masanobu was subsequently killed in 1572.  There are other theories concerning the year of his demise.

According to various genealogical records, Masanobu and the daughter of Kita Nobuchika were the parents of a daughter named Takadono-in who wed Shichinohe Naotoki.  Later, the daughter of Takadono-in became the wife of Kemanai Masatsugu.