He became chief in 1636, and was always friendly with the English settlers, aiding them in their wars against other Native tribes.
In 1642 one of his chiefs was attacked by the Mohican chief Uncas. To avenge the insult, Miantonomoh made war on Uncas with nearly 1,000 warriors, but was defeated and taken prisoner. Uncas took him to Hartford, Connecticut and told the English officers there that he would do with his captive whatever they commanded. Though Miantonomoh had made war with their consent, they advised that he should be killed. He was then taken back to Norwich, where he had been defeated, and killed by Uncas with a tomahawk (1643).
A monument was erected in 1841 on the place, which is still called Sachem's Plain.