Date's right eye was lost to smallpox as a five-year old. Despite being the oldest son, his mother felt he was therefore unfit to inherit the family title. The mother pressed for his younger brother Kojiro to become heir. So strong was her determination, she attempted to poison Date. Masamune killed his sibling to maintain his title and save his own life.

Following his father Termune's retirement, Masamune began conquering the surrounding domains. The neighboring Hatakemya clan protested these advances to the retired father who advised he had no control over his son's actions. The Hatakemyas kidnapped the father in an effort to stop Masamune. In the ensuing confrontation, the father shouted to Date to kill the kidnappers even if it meant the father would die. Masamune killed the kidnappers and all their families.

Masamune was a tough and resolute warlord, going on to serve brilliantly for Toyotomi Hideyoshi as a general in the Korean Campaigns as well as fighting for Tokugawa Ieyasu during his final rule of Japan. In later years, he founded the city of Sendai and brought it to great prosperity. Date Masamune died in 1636 at age 68.