When his older brothers, Masatsura KUSUNOKI and Masatoki KUSUNOKI, were defeated and killed by the army of KO no Moronao in the Shijonawate War in 1348, Masanori took over the family estate to be the head of family, and became a spearhead busho of the Southern Court (Japan).
正平(日本)3年/貞和4年（1348年）、四條畷の戦いにおいて兄楠木正行・楠木正時が高師直の軍に敗れて討死すると、楠木氏の家督を継ぎ、南朝(日本)の先鋒武将となる。