Born in 1585 as the son of Tozawa Moriyasu, Masamori's succession was initially questioned because of his mother's low status, but his retainers championed him and secured Toyotomi Hideyoshi's recognition as clan head. At the Battle of Sekigahara (1600) he fought for the Eastern army, distinguishing himself by capturing Tozenji Castle held by Uesugi forces in Dewa's Shonai region. For this he was granted 40,000 koku in Hitachi's Taga and Ibaraki districts, and in 1602 he thoroughly renovated the Otsuka clan's Ryushiyama Castle into a flatland-mountain castle, renaming it Matsuoka Castle and founding the Hitachi Matsuoka domain. He governed Matsuoka for the next twenty years, also serving in the Siege of Osaka, before being transferred in 1622 to Shinjo in Dewa Province with 60,000 koku following the dispossession of the Mogami clan, becoming the first lord of Shinjo domain. He died in 1648 at age 64.

Having overcome doubts about his succession through the loyalty of his retainers, he combined martial achievement with administrative skill, proving himself in castle-building and domain governance both at Matsuoka and later at Shinjo.

He rebuilt Ryushiyama Castle into the early-modern Matsuoka Castle and laid the foundation of the Hitachi Matsuoka domain. He later became the first lord of the 60,000-koku Shinjo domain, whose lordly line continued until the end of the Edo period.