- Born: 1749 (official date)
- Died: 1761 (original Mitsunaga); 1767 (second Mitsunaga/Yorisada)
- Other Names: 相良頼完 (Sagara Yorisada)
- Japanese: 相良晃長 (Sagara Mitsunaga)
Mitsunaga was adopted into the Sagara clan from the Akizuki clan in 1759, when the clan was lacking in direct heirs. He was a relative of the Sagara daimyô through a maternal connection, and was eight years old at the time; his age was officially reported as 11, however.
An unhealthy young man, Mitsunaga was blind in one eye, and quite close to it in the other. He died a mere two years later, at an official age of 13. The clan once again needed to procure an heir, but the Akizuki had no more sons they could adopt. The Sagara then inquired with the Washinoo family, court nobles based in Kyoto, whose daughters had married into the Sagara clan in the past; the Sagara and Washinoo also shared a connection to the Fujiwara clan, making them relatives, albeit extremely distant ones. Arrangements were successfully made for Washinoo Isomaru, a twelve-year-old young man who had just returned from some time in a monastery in Osaka, to replace the late Mitsunaga. He traveled to Edo pretending to be a Sagara retainer, and soon afterwards assumed the identity of Sagara Mitsumasa, daimyô of Hitoyoshi.
Domain documents record that the sick and blind Mitsunaga miraculously recovered from his illnesses in 1762 (the year he was replaced), and coincidentally developed more handsome features. Washinoo Isomaru, now known as Mitsunaga, later changed his name to Yorisada, and after reaching age 17, received an audience with the shogun. When he died two years later, the clan rushed to arrange for the adoption of an heir, though this time with less difficulty or secrecy.
- Roberts, Luke. Performing the Great Peace: Political Space and Open Secrets in Tokugawa Japan. University of Hawaii Press, 2012. pp88-89.
- By traditional age calculation, which counts the number of calendar years (including partial ones) in which the individual has lived.