Horsfield was born in Bethlehem, Pennsylvania, and studied medicine at the University of Pennsylvania. He was the grandson of Timothy Horsfield, Sr. (1708-1773), who was born in Liverpool and emigrated to New York in 1725. In New York, his brother Isaac and he ran a butcher shop. The Horsfield family converted from the Church of England to Moravianism, a Protestant denomination with a strong emphasis on education. In 1748, he applied for permission to reside in Bethlehem, Pennsylvania. He, however, moved only his family to Bethlehem and joined them the next year. When Northampton County was created in 1752, he was made a justice of peace by Governor Hamilton. In 1763 he was commissioned a colonel in the forces defending the frontiers against Indian raids. One of the sons, Joseph Horsfield was a delegate in the Pennsylvania convention to ratify the Federal Constitution. Grandfather Horsfield was a friend of Benjamin Franklin and finds mention in the latter's autobiography. Horsfield's father was Timothy Horsfield, Jr. (died April 11, 1789) and he married Juliana Sarah Parsons of Philadelphia in 1738. Thomas Horsfield was born in Bethlehem on May 12, 1773. He went to school at the Moravian schools in Bethlehem and Nazareth. He took an interest in biology and took a pharmacy course under a Dr Otto (probably John Frederick Otto MD, of Nazareth). In 1798, he graduated in medicine from the University of Pennsylvania, his thesis being on the effects of poison ivy.