Born in Woburn, Massachusetts, he attended local schools, before serving an apprenticeship and studying at Harvard University. Thereafter he was for a while a school teacher in Rumford (now Concord), New Hampshire, but in 1774 he was commissioned as a Major in the local regiment. After being briefly imprisoned on the correct, but unprovable, suspicion of supporting the British, he made his way to Boston where he remained until it was surrendered to the rebels in 1776 and he sailed to London. There he held a number of government posts and became acquainted with the President of the Royal Society Joseph Banks.
As Lieutenant-Colonel of the King's American Dragoons, he went back North America following the British defeat at Yorktown in 1781. However, after the conclusion of peace he returned, in 1783, to Europe where he entered the service of the Bavarian government, rising to be Minister of War, for which he was created a Count of the Holy Roman Empire.
From 1795 to 1796 and again from 1798 to 1802 he was in Britain. There he was active in a number of philanthropic causes and experimented on the nature of heat, arguing that it was a mode of motion. During the latter visit the he drew up the proposals to establish a practical scientific institution which, following a meeting at Banks's Soho Square house on 7 March 1799, was implemented as what became the Royal Institution. His especial contribution in the Institution's very early years was in overseeing the conversion of 21 Albemarle Street to make it fit for purpose. He abruptly left the Royal Institution in 1802, possibly because he had been embezzling funds, though this was never proved at the time or subsequently.
He left London for France during the Peace of Amiens. In 1805 he married Lavoisier's widow, but the marriage was not a success and they separated four years later. However, he remained in France and died at Auteuil.
Further reading: Oxford Dictionary of National Biography