In 1769, when seventeen, he was apprentice and clerk to a drygoods dealer at Boston. There he went to an evening-school to learn French, paying only for the hours he attended.
A note-book made by him about this time still exists. It abounds in caricatures. Has receipts for different fireworks. One of these ends with, 'Love is a noble passion of the mind.' Contains the sum he paid for learning French and for pew-rent, and the sums gained by cutting and carting firewood for relatives. Instructions for the back sword exercise, with a sketch of two combatants; and later there is an account of 'what expense I have been at towards getting an electrical machine,' and 'an account of what work I have done towards getting an electrical machine.'
In the winter of 1770 he was ill for five weeks with fever. Then for eighteen months off and on he boarded with Dr. John Hay, of Woburn, and whilst with him he learned something of anatomy, chemistry, materia medica, surgery, and physic. During the summer, 1771, he went to Cambridge, to attend Mr. Winthrop's lectures on Experimental Philosophy. In the winter of 1771-2 for some weeks he was teaching in a school at Wilmington, and in the spring he taught at Bradford. In the summer he left Dr. Hay for good, because he was asked by Colonel Walker to become the fixed master of a school at Concord, New Hampshire. This place had been called Rumford when it belonged to Essex County, Massachusetts. The name was changed when the disputes as to the county to which it belonged were ended.
The Rev. T. Walker was the first minister of Concord. He was a native of Woburn and connected with the Thompson family. He was the chief man in Concord. His son was a colonel and a lawyer, and his daughter, when about thirty, was married to Colonel Rolfe, who was sixty. She was left a rich widow in two years, and in the middle of the following year Thompson came as schoolmaster to Concord. He was not yet quite twenty. His friend Baldwin describes him 'as of fine, manly make and figure, nearly six feet high, with handsome features, bright blue eyes and dark auburn hair. His manners were polished and his ways fascinating, and he could make himself agreeable. He had well used his opportunities of culture, so that his knowledge was beyond that of most of those around him, and he was able to give satisfaction as a teacher.'