Tenshouin was born in Kagoshima in 1835. In 1853, she became the adopted daughter of Shimazu Nariakira. In August 21, 1853, she travelled by land from Kagoshima via Kumamoto to the Edo jurisdiction, never to return to Kagoshima again.
Atsuko was thought to be sent to Edo castle with the aim of helping Shimazu Nariakira politically. The question of the next heir to the Shogunate was divided between the choice of Tokugawa Yoshinobu, then head of the Hitotsubashi-Tokugawa house and Tokugawa Yoshitomi, then head of Kii-Tokugawa house and later known as Tokugawa Iemochi. In order to ensure that Yoshinobu becomes the next in succession, Atsuko was arranged to wed into the Tokugawa Clan.
In November, 1856, Atsuko married Tokugawa Iesada. In 1858, both Tokugawa Iesada and Shimazu Nariakira died. The 14th shogun was decided to be Tokugawa Iemochi. Following the demise of her husband, Atsuko took the tonsure, becoming a Buddhist nun, and took the name Tenshouin. In 1862, as part of the Koubu Gattai ("Union of Court and Bakufu") movement, Iemochi was married to Imperial Princess Kazu-no-Miya Chikako daughter of Emperor Ninkou, and younger sister of Emperor Koumei. The Satsuma clan brought up the request for Tenshouin to return to Satsuma, but was rejected by Tenshouin herself. In 1866, Iemochi died. Tokugawa Yoshinobu became the next shogun. During the Meiji Restoration, Tenshouin and Seikan'in (Kazu-no-Miya's name after tonsure) helped negotiate for the peaceful surrender of Edo Castle.