Dr. Paine returned to America in 1775, shortly after hostilities commenced, and while there was apparently no legal impediment to his return to Worcester, it was doubtless a very prudent decision of Dr. Paine not to make the attempt. His feeling of personal loyalty to the government was too strong to allow him even to appear to yield to the Revolutionists, then dominating his native town, and he wisely returned to England. His study of medicine there must have been pursued with unusual zeal and success, for Nov. 1775, he received from Marischal College, Aberdeen, the degree of M. D.
Soon after obtaining this distinction, he received an appointment as Apothecary to the British forces in America, and served in Rhode Island and New York till 1781, when he returned to England, in company with his patient, Lord Winchelsea. While in England, in 1782, he is said to have been made Licentiate of the Royal College of Physicians of London.
October 23, 1782, he was commissioned Physician to His Majesty's Hospitals within the district of North America, commanded by Sir Guy Carleton, and he reported for duty at Halifax, N. S. Letters which have been preserved show that during this year at Halifax he had won the respect, friendship and confidence, not only of his immediate medical superior, Dr. Nooth, but also of Lord Wentworth, Governor of the Province.
In the summer of 1784, Dr. Paine took possession of La Tete, an island in Passamaquoddy Bay, granted him by the British Government, for his services in the war. He remained there less than one year, and then made his residence in St. John, N. B., where he took up the practice of his profession. The cause of the removal from the island was the protest of his wife that the children could not receive a proper education in that isolated spot.
He was elected member of the Assembly of New Brunswick from the county of Charlotte, and was appointed Clerk of the House. He was commissioned as a justice for the county of Sunbury. There is abundant evidence of the high estimate placed on his character and ability in the numerous offices which he held during his residence here.
July 29, 1786, he wrote to a friend: "I do a great deal of Business in my Profession, but I get very little for it. The truth is we are all very poor, and the most industrious and economical gets only a bare subsistence. However, it will soon be better as the Province is daily filling with stock of all kinds."
In 1787 Dr. Paine made application for leave to visit and reside in New England while remaining on half pay, and a permit to that effect was issued by the War Office.
In Salem he devoted himself to the practice of medicine in the town where he had been known as a student of the famous Dr. Holyoke, and where his wife had spent her early life.
In 1793 his father died, and he removed to Worcester, and for the remaining forty years of his life he resided in the paternal mansion. His father's property was large, and as he was not an absentee, it was not confiscated. By his will it was equally divided between his children, the farm and homestead covered 1230 acres. Dr. Paine bought the shares of his brothers, and sisters in same for 2,000 pounds sterling, but the deeds were given to Nathaniel Paine in trust for William, for the doctor was as yet, but an alien in his native state. The year 1812 was a critical one, bringing a most important question for him to decide, for war arose between Great Britain and the United States, and he was still a half-pay officer in His Majesty's service. He therefore resigned from the British service, and in 1812 petitioned the Legislature for its consent to his being a naturalized citizen of the United States.
William Paine was one of the founders of the American Antiquarian Society of Worcester. His name was omitted from the act of incorporation because he was an alien. The next year, 1813, he was elected Vice President of same.