He fought alongside his father, Shimazu Hisatsune, in the 1281 defense against the second Mongol invasion, and became clan head upon Hisatsune's death in 1284. Around that time, either Hisatsune or Tadamune became the first Shimazu head to relocate from Kamakura and take up permanent residence in Kagoshima. From then on, the Shimazu would administer their territory relatively directly, rather than appointing representatives (shugo dai) to administer in their stead.
Tadamune was succeeded as clan head by his eldest son Shimazu Sadahisa, while his other five sons founded branch families: Shimazu Suketada was progenitor of the Hongô clan (aka Miyakonojô Shimazu), Shimazu Sukehisa was progenitor of the Kabayama clan, Shimazu Tokihisa was progenitor of the Niiro clan, Shimazu Tadamitsu was progenitor of the Sata clan, and Shimazu Tadauji was progenitor of the Izumi clan.
|Head of Shimazu clan & jitô-shugo of Satsuma province