Born in Pennsylvania, he was a son of William Magaw of Shippensburg. His brothers were Dr. William Magaw and Colonel Robert Magaw, a colonel in the American Revolutionary War. He was member of first class graduated from the College of Philadelphia in 1757, when he received the Bachelor's Degree; the Master's degree was conferred in course in 1760. He was educated for a tutorship at the suggestion of the said College authorities and later studied divinity, and went to England for orders and ordained as an Anglican priest in 1767. On a return he became a missionary of the Society for Propagation of the Gospel at Dover and Duck Creek, Delaware, and was appointed Rector of St. Paul's Church in Philadelphia, in 1781, and held the office until 1804. He received the honorary degree of Doctor of Divinity from Pennsylvania in 1783. Dr. Magaw was Vice-Provost and Professor of Moral Philosophy in the University from 1782 to 1791 and when the union of the College and University occurred in 1791 he was the only professor in the Faculty of the latter who was not included in the new combined body, his withdrawal, which was voluntary, being due to his desire that his friend, Dr. John Andrews, should be elected to his place. He assisted Rev.James Abercrombie, D.D., in founding the Academy of the Protestant Episcopal Church in Philadelphia. He was elected a member of the American Philosophical Society in 1784. Numerous sermons that he preached on special occasions have been published. Bishop White, in his memoirs of the said church, makes complimentary mention of the part Dr. Magaw had taken in 1784 in the organization of the Protestant Episcopal Church in the United States. Dr. Magaw married Lucia, daughter of Andrew Doz, of Philadelphia.