Treat was born in Somerset County, England on February 23, 1622, but was brought to Massachusetts as a child. His family were among the early settlers at Wethersfield, Connecticut. He settled in Milford, Connecticut in 1639 and became one of the leaders of the New Haven Colony, serving in the General Court as its assembly was known.
On Christmas day, December 25, 1647 he married Jane Tapp in Milford. The couple would have eight children. Their great-grandson, Robert Treat Paine, wold go on to sign the Declaration of Independence.
When the Connecticut Charter of 1662 forced the New Haven Colony to merge with Connecticut in 1765, Treat led a group of dissidents who left the colony. They moved to New Jersey and founded the town of Newark in 1666. Robert himself returned to Milford in 1672 and lived there the rest of his life.
Treat headed to colony's militia for several years, principally against the Narragansett Indians. This included participating in King Philip's War in 1676. He served on the Governor's Council continuously from 1676 to 1708. He was first elected Governor in 1683.
Sir Edmund Andros supplanted him in 1687 , and attempted to make Connecticut part of the Dominion of New England. Treat is credited with having a role in concealing the state's Charter in the Charter Oak, and resumed his job as governor when the dominion scheme fell apart in 1689. He was re-elected annually until 1698. Robert died in Milford on July 12, 1710.