Sanada Masayuki, born in 1547 as the third son of Sanada Yukitaka, possessed no inheritance rights due to his elder brothers Nobutsuna and Masateru. At age seven, he became a Takeda clan hostage, entering Takeda Shingen's inner circle of young servants where his strategic talents emerged. Later adopted by the Mutō family, he took the name Mutō Kihei.

Masayuki fought for the Takeda in pivotal conflicts: the Fourth Battle of Kawanakajima (1561), Battle of Mimasetōge (1569), and Battle of Mikatagahara (1573). Following his brothers' deaths at Nagashino (1575), he inherited Sanada clan leadership with Takeda Katsuyori's endorsement. In 1580, he seized Numata Castle from the Hōjō clan, earning the title "Awa-no-kami" (Prefect of Awa).

After the Takeda collapse in 1582 and Oda Nobunaga's death, Masayuki maneuvered through the Tenshō-Jingo power vacuum by shifting allegiances among Tokugawa, Uesugi, and Hōjō clans. He retook Numata Castle via tactical betrayals and deployed his son Nobuyuki to hold Iwabitsu Castle. His 1585 victory over Tokugawa forces at Kami River cemented his renown as a strategist.

During the Keichō era (1600), Masayuki and his son Yukimura defended Ueda Castle against Tokugawa Hidetada's superior army. Employing psychological warfare and terrain mastery, they executed unconventional tactics—including pouring boiling porridge during feigned surrender talks—delaying Hidetada’s advance to Sekigahara. This siege exemplified Masayuki’s commitment to Sanada autonomy through military ingenuity and calculated alliances amid Tokugawa unification efforts.