Shirasaka Shimōsa-no-suke served as a bushi during the Sengoku period. He was a retainer of the Kitahara clan of Hyūga Province.
Shimōsa-no-suke was born as the son of Shirasaka Shimōsa-no-kami Kanetsugu.
His father, Kanetsugu, served as the lord of the manor in Takahara and Takasaki as well as the lord of Shiwachi Castle, but died in battle against the Hongō clan and Shiwachi Castle was taken away. Meanwhile, his son, Shimōsa-no-suke, is referred to in historical accounts as an individual from Takasaki, so it is surmised that, after the death of his father, he moved away from Shiwachi.
In 1558, Kitahara Kanemori, the thirteenth head of the Kitahara clan, suddenly died, and then his daughter who was supposed to wed the lineal heir of Kitahara Kanetaka, also died. This triggered a succession struggle. Kanemori’s formal wife, Asō, was the daughter of Itō Yoshisuke, so the Hyūga-Itō clan intervened and plotted to have Asō re-marry with Mangata Uemon-no-suke of an illegitimate branch of the Kitahara. This would prevent the Kitahara family from inheriting the headship of the clan.
After Shimōsa-no-suke and others voiced opposition to this plan, on 3/17 of Eiroku 2 (1559), Yoshisuke summoned Shimōsa-no-suke and members of the opposition faction to Tonokōri Castle for questioning. Yoshisuke then had soldiers lay in ambush along the road home in Mutsubaru. As Shimōsa-no-suke and the others passed-through this area, they were surrounded, whereupon eleven individuals including Kitahara Mikawa-no-kami were forced to commit seppuku. Shimōsa-no-suke managed to flee to the territory of Kabayama Yoshihisa, a retainer of the Shimazu, but members of the opposition were purged so Mangata Uemon-no-suke took over the headship of the clan. By this means, the Kitahara were usurped by the Itō clan.
Shimōsa-no-suke made plans for the revival of the Kitahara family, and via Kabayama Yoshihisa, requested support from Shimazu Takahisa, the military governor of Satsuma, Ōsumi, and Hyūga provinces. After a positive response from Takahisa, Shimōsa-no-suke further requested the cooperation of Hongō Tokihisa and Sagara Yoshihi. In 1562, the Shimazu, the Hongō, and the Sagara expelled the Itō clan from Masakiin and reclaimed the former territory of the Kitahara. Kitahara Kanechika inherited the headship of the clan and entered Iino Castle. Shimōsa-no-suke also entered Takahara Castle, thereby placing in check the Itō.
Nevertheless, in 1563, the Sagara betrayed the Kitahara by joining forces with the Itō and toppled Daimyōjin Castle. In the fifth month of 1564, Itō forces felled Ima Castle, the base of the Okobira clan who served the Kitahara. Fearing consequences, Shimōsa-no-suke absconded to the Kitahara clan. His subsequent whereabouts are unknown but, according to one account, he turned for protection to Kabayama Yoshihisa and may have served him thereafter.