He was an army officer from 1757 until ill health induced him to sell his lieutenant-colonelcy in 1769. Commissioned by the Crown as governor of North Carolina early in 1771, he sailed from Long Island in July and took the oath of office before the council at New Bern on Aug. 12. Though reports of his amiable character preceded him, he soon became involved in protracted conflicts with the sensitive assembly, first, over the sinking-fund tax, whose discontinuance in 1771 he disallowed as illegal and violative of public faith; and, beginning in 1773, over the right of the courts to attach property in North Carolina for debts of non-residents to North Carolinians. The assembly, dominated by the eastern planters and merchants, would pass no new court law without the "foreign attachment clause"; and Martin, who was under positive instructions from the Crown, would not assent to a law containing the clause. Consequently, the judicial system of the colony collapsed in 1773, and the ensuing confusion and resentment was accentuated by the emergency creation by royal prerogative of criminal courts whose expenses the assembly in December refused to bear. The sinking-fund tax was not collected generally, and the province remained without courts for the trial of civil cases involving more than £20. The survey of the North Carolina-South Carolina boundary line in 1772, as decreed by the Crown, deprived the colony of much claimed territory and created dissatisfaction. In bold defiance of the governor, the Patriot leaders convened at New Bern in August 1774 a revolutionary provincial congress which elected delegates to the first Continental Congress and inaugurated a system of county committees of safety which gradually superseded the royal government as the source of authority. With his authority and influence gone and fearing personal violence from the local militia after the battle of Lexington, Martin fled from New Bern, arriving at Fort Johnston on June 2, 1775. In July he was driven aboard a British vessel in the Cape Fear River. Martin sought to become informed of conditions in the colony, to eliminate abuses in administration, and to pacify the Regulators, but he was not able to reconcile the tempers, aims, and political philosophies of colony and mother country. Aboard ship in the Cape Fear, he formulated a plan for the subjugation of the Southern colonies which was approved by the British government; but the Loyalist Scotch Highlanders, assembled under his direction, were defeated at Moore's Creek Bridge, February 27, 1776, before the British reinforcements arrived off the Cape Fear. In May, Martin departed with the British for an attack on Charleston; he returned in the summer to "Rockhall, " and in 1779 joined the Clinton expedition against South Carolina, serving with usefulness and credit as a volunteer with Cornwallis in the campaign of 1780-81 in the Carolinas. Declining health caused him to leave Cornwallis at Wilmington in April 1781, and sail via Long Island for London, where he died in the spring of 1786. He drew his salary as governor until October 1783, and was granted compensation for his confiscated North Carolina property by the American Loyalist Claims Commission, before which he testified in behalf of the claims of many North Carolina Loyalists.
Though a military man without previous political experience, somewhat stubborn and insistent on prerogative, and unappreciative of the colonial position, Martin was accomplished, energetic, able, honest, faithful, as well as sincere and patient in his efforts to promote the public welfare and to conciliate the colony without violating his positive instructions and his conception of the duties of his office.
In 1761 he married his cousin Elizabeth, daughter of Josiah Martin at whose country seat, "Rockhall, " on Long Island he resided at various times. To this union were born eight children.