The colonies managed to prosper while Britain's mercantilistic policies were in force, but did so in spite of the policy rather than because of it. They violated British policy with abandon and thereby managed to prosper; otherwise they would have suffered severely.
Mercantilism was intended to provide a favorable balance of trade between the mother country and the colonies. British economists envisioned that the colonies would be a source of raw materials for Britain and also a market for manufactured goods. As a result the several Navigation Acts were passed which prohibited the colonies from trading with other countries or shipping on any ships other than those sailing the British flag. The Navigation Act of 1651 had been aimed at the Dutch who were England's rivals for the Atlantic trade. The problem for the colonists was that strict enforcement of the policy would result in an unfavorable balance of trade for them and a scarcity of hard money. The colonists ignored the law and conducted a lively and thriving trade with the Dutch and other countries. Among those who became quite wealthy from smuggling was John Hancock, of the famous signature. Several other Navigation Acts were subsequently passed, including the Staple Act and the Plantations Act which required all goods be landed in England and a duty paid before they could be shipped elsewhere. This Act, however, came in force during the English Civil War and the Protectorate when turmoil at home prevented the British from enforcing the policy.
Ultimately, customs agents were sent to the colonies to enforce the Acts, but they caused bitter resentment. The continued abuse of the Navigation Acts led to the cancellation of the Charter of the Massachusetts Bay Company. Subsequent attempts to enforce the acts came to naught, as colonial juries uniformly refused to find smugglers guilty.
Ultimately, with the ascension of George I and II to the throne, Parliament began a policy which Edmund Burke called "a wise and salutary neglect." British officials were busy protecting their own interests and the colonies were largely ignored. So the colonists prospered under mercantilism, but only because of their continuous and wilful violation of the policy.