Since his brothers Masatsugu SASSA and Magosuke (Naritsune) SASSA were killed in the war one after another, Narimasa took over the family estate and became the lord of the Hira-jo Castle in 1560.
兄に佐々政次（政次）、佐々孫介（成経）がいたが、相次いで戦死したため、永禄3年（1560年）に家督を継ぎ、比良城主となる。