Born in 1511 as the second son of Yamana Muchitoyo, shugo of Tajima. He succeeded to headship of the Yamana clan in 1528 upon the death of his uncle, Yamana Nobutoyo. At the time the Yamana house was split between the Tajima and Inaba shugo lines, but in 1548 Suketoyo defeated his cousin Yamana Nobumichi, the Inaba shugo, and reunified the clan. Within Tajima he also focused on developing the Ikuno silver mine, discovered in 1542, undertaking a major renovation of the ancestral Ikuno Castle to strengthen his economic base. He expanded and fortified Konosumiyama Castle, the Yamana clan's traditional headquarters and shugo seat, but in August 1569 an invasion force under Kinoshita Hideyoshi (the future Toyotomi Hideyoshi), acting on Oda Nobunaga's orders, captured Konosumiyama Castle along with seventeen other Tajima strongholds, forcing Suketoyo to flee to Sakai. He was later allowed to return through the mediation of the wealthy Sakai merchant Imai Sokyu and reconciled with Nobunaga, building a new stronghold, Arikoyama Castle, around 1574. In April 1580, however, a renewed Tajima invasion led by Hashiba Hidenaga took Arikoyama Castle on May 16, and Suketoyo died of illness within the castle on May 21 at the age of 70.

A Sengoku daimyo who showed the resolve to forcibly reunify the divided Yamana clan, while also devoting attention to strengthening his economic base through ventures such as the Ikuno silver mine. His reconciliation with Nobunaga also reveals a flexible ability to read the shifting tides of the age.

The ruins of Konosumiyama Castle, together with Arikoyama Castle, are jointly designated a National Historic Site as the "Yamana Clan Castle Ruins," and remain a valuable testament to the rise and fall of the Yamana clan and the Sengoku history of Tajima, drawing visitors to this day.