Uncas was born near the Thames River in present-day Connecticut, the son of the Mohegan sachem Owaneco.

In 1626, Owaneco arranged for Uncas to marry the daughter of the principal Pequot sachem Tatobem to secure an alliance with them.

Tatobem was captured and killed by the Dutch in 1633; Sassacus became his successor, but Uncas felt that he deserved to be sachem.

His failed challenges resulted in Uncas having little land and few followers, but Uncas saw that the newly arriving Puritan colonists, though few in number, had better weapons and much courage, so he started to develop a new strategy and alliance to work towards his ultimate goal of Grand Sachem.

He was a trusted ally of Captain John Mason, a partnership which lasted three and a half decades and several family generations beyond.

Uncas sent word to Jonathan Brewster that Sassacus was planning to attack the colonists on the Connecticut River.

The treaty dictated that Uncas could pursue his interests in the Pequot country only with the explicit approval of the Connecticut Colony.

This marriage gave Uncas some type of control over their land which he promptly sold to New England colonists.

Uncas' brother Wawequa, leading the pursuit, caught up to Miantonomo and struck him a fatal blow to the back of his head with a tomahawk.

Author James Fenimore Cooper portrayed a fictional Uncas as having made the leap over the falls in his 1826 book The Last of the Mohicans.

Narragansett sachem Pessachus proposed to go to war to avenge the death of Miantonomo, but the colonists promised to support the Mohegans.

The New England Confederation pledged any offensive action required to preserve Uncas in "his liberty and estate".