Suwa Tadamasa was the eldest son of Suwa Tadamichi and became "daimyō" in 1840 on the retirement of his father. He was strongly supported by his uncle, Matsudaira Sadanobu and rose to the post of "wakadoshiyori" in 1860, "jisha-bugyō" in 1862 and "rōjū" in 1864. As this was during the tumultuous Bakumatsu period, during which time the Tokugawa shogunate was forced to sign treaties with the Western powers to end Japan's self-imposed national isolation policy, Yorimasa played a role in the negotiations. Together with fellow "rōjū", Makino Tadayuki of Nagaoka Domain, he strenuously opposed the Second Chōshū expedition, and resigned his posts when the shogunate proceed with the punitive campaign against Chōshū Domain. In 1868, he also retired as "daimyō" in favor of his nephew, Suwa Tadaaya.