Born as the second son of the second shogun Tokugawa Hidetada, he became a shogun in 1623 when his father retired and initiated the cloistered rule as Ogosho lasting until 1632. To ensure the power of the shogunate, he eliminated a number of daimyos and established the central administration, lasting for the next 200 years until the Meiji Era. He established the alternating residency and hostage system (Sankin Kotai) in 1635, set the isolation policy, or Sakoku eventually closing off Japan from the rest of the world between 1633 and 1639, and saw the final eradication of Christians on the Japanese islands. He also completed anti-Christian policies; he executed or expelled all of remaining Christian missionaries and required the entire Japanese to register them to Buddishm temples. When the rebel againt this policy rose in the Shimabara, he supressed it harshly. The main domestic and foreign policies of the Tokugawa shogunate was completed by him.
Iemitsu was succeeded after his death by his eldest son Tokugawa Ietsuna in 1651.