Mishuku Masatsuna served as a bushō from the Sengoku to early Edo periods.

According to one theory, the Mishuku were an illegitimate branch of the Katsurayama clan which, in turn, descended from the Ōmori clan of Suruga Province.  The surname of Mishuku is said to have been derived from a reference to the lodge of Minamoto no Yoritomo in Aizawa near Mount Fuji in Suruga that was used by Katsurayama Koreshige in the late twelfth century for hunting in the summer.  

In 1568, Masatsuna was born as the second son of Mishuku Tomotsuna, a bushō and physician to Takeda Shingen, the sengoku daimyō of Kai Province.  Tomotsuna was a nephew of Katsurayama Ujimoto, a kokujin, or provincial landowner, in the Suntō District of eastern Suruga.  This relationship evidences the familial ties between the Mishuku and Katsurayama clans. 

Masatsuna first served Takeda Shingen and Takeda Katsuyori.  After the decimation of the Kai-Takeda clan in the third month of 1582, Masatsuna and his father turned to the Gohōjō clan, becoming retainers of Itabeoka Kōsetsusai for a stipend of 22 kan (1 kan = 1 to 2 koku).

In 1590, after the Gohōjō fell to the Toyotomi in the Conquest of Odawara, Masatsuna served Matsudaira Tadateru.  In 1616, Tadateru was removed from his position in the wake of the Siege of Ōsaka.  Masatsuna’s subsequent movements are unknown.  He either served another lord or returned to farming.

He died in Shinano Province.  He was fifty-six years old.