He was born to a branch family of the Shimazu clan, the Mimasaka Shimazu family (伊作島津家) but after his father Shimazu Yoshihisa died, his mother married Shimazu Unkyu of another branch family, the Soshū(相州家). Tadayoshi thus came to represent two families within the larger Shimazu clan.
Shimazu Katsuhisa, who presided over the Shimazu family, did not have a son and he was driven out by Shimazu Sanehisa, who was the head of yet another branch, the Sasshū (薩州家). Sanehisa then laid claim to be the head of the clan without being properly recognized by the rest of the families. Katsuhisa asked Tadayoshi for help to regain his position, and Tadayoshi sent his son Shimazu Takahisa to be adopted by Katsuhisa. In 1526, Katsuhisa handed over the position of the head of the family to Takahisa. In 1539 though, during the Battle of Ichirai, Tadayoshi defeated Katsuhisa (who would regain power later) and Takahisa came to be recognized by all members of the Shimazu clan as the head.
After Takahisa's succession, Tadayoshi retired to Kaseda in Satsuma Province. He held a great amount of power, trading with the Ryūkyū Kingdom and Ming Dynasty China. He also arranged for massive purchases of arquebuses to make the clan prosperous for the planned unification of Kyūshū by Takahisa.
Tadayoshi wrote an Iroha poem that sung of the importance of unity and also to give more literacy to his men. It begins with following words:
- Inishie no Michi wo Kikitemo Tonaetemo Waga Okonai ni sezuba Kahinashi
It means, "Even if you learn old ways, if you cannot use them as your own, it is meaningless." It was based on Confucianism and his educational philosophy would deeply influence his four grandsons, Shimazu Yoshihisa, Shimazu Yoshihiro, Shimazu Toshihisa, and Shimazu Iehisa. This would eventually make it way into modern philosophies in the Meiji period as Satsuma han took part in modernizing Japan.
Tadayoshi called himself Shimazu Jisshinsai (島津日新斎) in later years and praised his four grandsons as "Yoshihisa the Leader," "Yoshihiro the Brave," "Toshihisa the Planner", "Iehisa the Tactician" and later as "Nisshin Dai Bosatsu", which will be greatly revered by Satsuma Samurai. Tadayoshi died in 1568 at the age of 77.