Asahina Yasutomo served as a retainer of the Imagawa clan and as lord of Kagegawa Castle in Tōtōmi Province. Yasutomo was the son of Asahina Yasuyoshi. Yasuyoshi’s formal wife was the daughter of Nakamikado Nobuhide. She was a niece of Jukeini, the formal wife of Imagawa Ujichika and mother of Imagawa Yoshimoto. Yasutomo and Yasuyoshi had close relations with Yamashina Tokitsugu, a kuge, or court noble who served the Imperial family and maintained relations with numerous daimyō. In 1556, Tokitsugu presented Yasutomo with an anthology of one hundred songs written at the Sanzenin, a temple for the Tandai sect in Kyōto.
Following the death of Yasuyoshi, Yasutomo became head of the family and received the Court title of Provincial Governor of Bitchū. In 1558, he restored the Ryōzen Temple in the Suntō District of Suruga Province. In 1560, Yasutomo joined Ii Naomori in attacking the Oda at Washizu fortress as part of a larger invasion of Owari Province by Imagawa Yoshimoto. Yasutomo helped defend Ōdaka Castle, but Yoshimoto then perished in the Battle of Okehazama. This compelled Yasutomo to yield the castle and retreat.
The unexpected loss of Yoshimoto led to disarray within the provinces of Mikawa and Tōtōmi held by the Imagawa, causing disaffection among the retainers and breaking support for Imagawa Ujizane, the eldest son and heir of Yoshimoto. In 1562, Ono Michiyoshi falsely claimed that Ii Naochika plotted a rebellion, causing Ujizane to order his execution. During the Eiroku era (1558-1570), Yasutomo joined Miura Ujimitsu in negotiations with the Uesugi clan of Echigo Province.
In 1568, Takeda Shingen of Kai Province unilaterally rejected an alliance and invaded Suruga Province. This caused Ujizane to flee Suruga whereupon Yasutomo offered him protection at Kagegawa Castle in Tōtōmi. Toward the end of the year, Tokugawa Ieyasu invaded Tōtōmi, capturing Hamamatsu Castle and extending his control into the province. His forces then laid siege to Kagegawa Castle. This led to a majority of the former retainers of the Imagawa to betray Ujizane in favor of either the Kai-Takeda or Tokugawa clans. Yasutomo, however, continued to remain loyal to Ujizane and the Imagawa clan until the end.
For a period of five months, Yasutomo fought valiantly in an effort to defend the castle, but with the prospect of any reinforcements, hopes for victory dimmed. In mid-1569, Ujizane accepted a demand to turn over the castle, and retreated to Izu Province. Yasutomo accompanied Ujizane and traveled with him to Izu. Ujizane came under the protection of the Hōjō clan, while Yasutomo sought support from the Yamayoshi clan who served as retainers of Uesugi Kenshin from Echigo. In 1571, Ujizane obtained permission from Ieyasu to go to Hamamatsu Castle, but Yasutomo did not join him. Yasutomo’s whereabouts were unknown thereafter. Under one account, he may have served Sakai Tadatsugu, a retainer of the Tokugawa based in Mikawa Province.