Tōyama Yasumitsu served as a bushō during the Sengoku and Azuchi-Momoyama periods. He was a retainer of the Gohōjō clan of Sagami Province.
Yasumitsu was born as the second son of Tōyama Naokage, a retainer of the Gohōjō.
Yasumitsu is commonly understood as the younger brother of Tōyama Tsunakage. Based on the facts that he used the name of Asakura Naoyoshi, is surmised to have been twenty years younger than Tsunakage, and the Tōyama family was affiliated with the Edo group led by Tsunakage while only Yasumitsu affiliated with the Odawara group and was granted landholdings in the central portion of Sagami Province, it is noted that Yasumitsu may not have been an original member of the Tōyama family and instead was given the family name as an adopted son-in-law.
Yasumitsu was given one of the characters from the name of Hōjō Ujiyasu and promoted to serve as his attendant. After the dissolution of the three-way alliance between the Takeda, the Hōjō, and the Imagawa, despite having already transferred headship of the clan, Ujiyasu spearheaded negotiations with the Uesugi clan for which Yasumitsu served as a diplomatic go-between. Although the Hōjō and Uesugi formed an alliance, it did not proceed well.
In the tenth month of 1571, Ujiyasu died of illness. Ujimasu, who originally opposed making enemies of the Takeda clan, spurned the alliance with the Uesugi while, owing to his role as the intermediary, Yasumitsu was demoted and backslid to be under Hōjō Saburō who had been sent to the Uesugi as a hostage at Kasugayama Castle. After marrying the niece of Uesugi Kenshin, Saburō adopted the name of Uesugi Kagetora, becoming a designated heir of Kenshin. Yasumitsu served as a close associate of Kagetora. The death of Kenshin in 1578 led to the outbreak of a succession struggle known as the Otate Conflict. At this time, Yasumitsu called upon the Gohōjō to send reinforcements with the intention of fighting for Kagetora, but owing to the onset of the winter season, the Hōjō forces withdrew. With the aid of the Gohōjō, Yasumitsu attempted to flee to Odawara Castle, but, after encountering a rebellion by Naoe Munechika, the lord of Same-ga-o Castle, Kagetora took his own life and Yasumitsu followed in kind.