John Thomas was born in Marshfield, Massachusetts, in 1724, the son of John and Lydia Thomas. He studied medicine with Dr. Simon Tufts of Medford and later moved to Kingston where he practiced medicine. During the 1740s and 1750s, Thomas participated in three military expeditions to Nova Scotia. He was appointed a general officer by the Provincial Congress of Massachusetts in February 1775 and in June of that year was made a brigadier general by the Continental Congress. Under George Washington's command, Thomas's troops occupied Dorchester Heights on the night of 4-5 March 1776, forcing the British evacuation from Boston two weeks later. After this success, Thomas was promoted to major general and sent to Quebec, where he found his Continental troops outnumbered and suffering from smallpox. Thomas began a withdrawal of his troops and died of smallpox near Chambly, Quebec, on 2 June 1776.