He was born in Fushimi castle, the sixth son of Matsudaira Sadakatsu, who was half-brother to Tokugawa Ieyasu. In 1633, Sadamasa became a page (koshô) under Shogun Tokugawa Iemitsu; the following year he was granted a 5,000 koku fief, and was made head of the pages. He was then granted the 7,000 koku domain of Nagashima han in Ise province the following year (1635), and the year after that, continued as an attendant to the shogun.
Sadamasa was made lord of Kariya han, a 20,000 koku domain, in 1649.
When Shogun Iemitsu died in 1651/4 and was succeeded by his ten-year-old son, Tokugawa Ietsuna, a group of Elders led by Hoshina Masayuki took on the responsibility of overseeing shogunate affairs and policy until the young shogun came of age. In the seventh month of that year, Sadamasa sent a missive to the shogunate formally protesting their policies. He asserted his loyalty to the shogun but declared that he could not serve under a ruling council that governed in such a way, and that he had taken the tonsure. Sadamasa offered to return his lands - Kariya domain - to the shogunate, and suggested that the wealth he was thus returning could be spent to help bolster the income of poor hatamoto.
The ruling council declared Sadamasa insane, attaindered him (took back his domain so that another daimyô could be enfeoffed in those lands), and placed him into the care of his older brother, Matsudaira Sadayuki, lord of Matsuyama han on Shikoku, where he lived out the rest of his life in forced retirement.
- "Matsudaira Sadamasa." Asahi Nihon rekishi jinbutsu jiten 朝日日本歴史人物事典. Asahi Shinbun shuppan.
- "Matsudaira Sadamasa." Digital-ban Nihon jinmei daijiten デジタル版 日本人名大辞典. Kodansha, 2009.
- Roberts, Luke. Performing the Great Peace: Political Space and Open Secrets in Tokugawa Japan. University of Hawaii Press, 2012. p77.
- Luke Roberts gives 1662 as the date of Sadamasa's death, while encyclopedias such as the Asahi Nihon rekishi jinbutsu jiten, Sekai daihyakka jiten, and Nihon jinmei daijiten give either 1672 or 1673 as the year. The Asahi Nihon rekishi jinbutsu jiten gives the date of his death as 1673/1/11, though the Nihon jinmei daijiten gives the date as 11/24.