Shimodaira Yoshinaga served as a bushō during the Sengoku and Azuchi-Momoyama periods. He was a retainer of the Uesugi clan and the lord of Senzu Castle in the township of Tsumari in the Uonuma District of Echigo Province.
From 1554 to 1556, Yoshinaga engaged in a dispute with Ueno Ienari in regard to the territory of Ueda. This escalated so that Ōkuma Tomohide (backing the Shimodaira) came into conflict with Honjō Saneyori (backing the Ueno). During this period,
Owing to ongoing disputes among his band of retainers, revolts by kokujin, or provincial landowners, and a stalemate in battle against the Takeda clan, Yoshinaga’s lord, Nagao Kagetora (later known as Uesugi Kenshin), became disillusioned with the state of affairs and holed-up in the Bishamonten Hall (Vaiśravaṇa) and cultivated an interest in religion. On 3/23 of Kōji 2 (1556), he informed his band of retainers of his intention to enter the priesthood and, on 6/28, fled Kasugayama Castle to travel to Mount Kōya. On 8/17, his retainers caught-up with him in the village of Handagō in the Katsujō District in the foothills of Mount Katsujō in Yamato Province. After earnestly pleading with him, Kagetora agreed not to enter the priesthood. This is known as the shukkei sōdō, or the Priesthood Disturbance.
On 7/5 of Eiroku 7 (1564), Nagao Masakage, the head of the Ueda-Nagao clan and lord of Sakado Castle in Echigo, drowned in the Nojiri pond near Sakado Castle. He was thirty-eight years old. As to the reasons for the drowning, there is a theory that, in a drunken state, he fell out of a boat while enjoying merriment. Alternatively, upon orders of Kenshin, he was murdered by Yoshinaga or by Usami Sadamitsu. In any event, the details are uncertain. In an account from the mother of a retainer, Kokubun Hikogorō, who was on the boat with Masakage, there was a wound below the shoulder on his recovered remains. Hikogorō is said to have also died in this incident.
In 1578, during the succession struggle in the Uesugi clan known as the Otate Conflict, Yoshinaga supported Uesugi Kagetora. In its aftermath, his landholdings were seized and granted to Komorizawa Masahide.