In 1647, Mason assumed command of Saybrook Fort which controlled the main trade and supply route to the upper river valley. The fort mysteriously burned to the ground but another improved fort was quickly built nearby. He spent the next twelve years there and served as Commissioner of the United Colonies, as the chief military officer, magistrate, and peacekeeper. He was continually called upon to negotiate the purchase of Indian lands, write treaties, or arbitrate some Indian quarrel, many of which were instigated by his friend Uncas. His leadership abilities were unrivaled, which prompted the New Haven Colony to offer him a very lucrative position as manager of their enterprise in relocating to the Delaware River area. However, he declined the offer and remained in Connecticut.