Born as a son of Koga Kubo Ashikaga Masauji, he entered the priesthood early and served as a deputy abbot at Tsurugaoka Hachimangu. When his father and elder brother Takamoto clashed during the Eisho Rebellion, he returned to secular life. On October 15, 1517, with backing from the Kazusa Takeda clan, he seized Oyumi Castle in Shimosa Province from the Chiba clan's senior retainer Hara Tanetaka. The following year he proclaimed himself "Oyumi Kubo" in opposition to his brother, the Koga Kubo. On October 7, 1538, he died in battle against Hojo Ujitsuna at the First Battle of Konodai, alongside his younger brother Motoyori and heir Yoshizumi.

A bold figure who leveraged his Ashikaga lineage to proclaim himself a rival Kubo. His alliance-building across the Boso Peninsula and direct confrontation with the rising Later Hojo clan reflect the meritocratic power struggles of the Sengoku era.

He established the independent authority of the Oyumi Kubo, becoming an emblematic figure of the Sengoku-era turmoil in the Boso Peninsula. After his death, Oyumi Castle reverted to its former lords, the Hara clan, who rebuilt it as Oyumi (Shomu) Castle.