When his father Toyoyuki was killed in 1471, his family, relatives, and vassals were thrown into confusion over the post of Shugo, and in that confusion Masayuki competed for the post of Shugo with Motoyuki YAMANA who was his own uncle and had already taken the post of Shugo before him.
父の豊之が文明3年（1471年）に殺害され、守護職を巡って家中が混乱する中、政之は自身の叔父で先に守護に就任していた山名元之と守護職を巡り争った。