Benjamin Thompson was the son of a farmer from Woburn, Massachusetts. He started his career apprenticed to a merchant but was injured in a fireworks accident and moved to Boston. In 1772 he married a rich widow and moved to live in Rumford (now Concord, New Hampshire). When the American Revolution broke out, he took the English side and spied for them. By 1775 the hostility of his countrymen toward him had grown to such a pitch that he was forced to sail to England, leaving his wife and daughter behind. Once in England, his opportunist nature quickly raised him to the position of colonial undersecretary of state but, with the end of the American Revolution, he moved to Bavaria.

Here he rose rapidly to high government administrative positions and initiated many social reforms, such as the creation of military workhouses for the poor and the introduction of the potato as a staple food. In 1790 he was made a count in recognition of his service to Bavaria, taking the name of his title from the town of Rumford, New Hampshire.

It was in Bavaria that he first became interested in science, when he was commissioned to oversee the boring of cannon at the Munich Arsenal. Rumford was struck by the amount of heat generated and suggested that it resulted from the mechanical work performed.

According to the old theory of heat, heat produced by friction was caloric ‘squeezed’ from the solid, although it was difficult to explain why the heat should be released indefinitely. Rumford, in his paper to the Royal Society An Experimental Enquiry concerning the Source of Heat excited by Friction (1798), suggested the direct conversion of work into heat and made quantitative estimates of the amount of heat generated. It was suggested that the heat came from the lower heat capacity of the metal turnings, although Rumford could discount this by using a blunt borer to show that the turnings produced were not important. Another objection – that the heat came from chemical reaction of air with the fresh surface – was disproved by an experiment of Humphry Davy (1799) in which pieces of ice were rubbed together in a vacuum. The idea that heat was a form of motion replaced Lavoisier's caloric theory over the first half of the 19th century.

Rumford returned to London in 1798 and there began work on a series of inventions, including a kitchen stove, a photometer, and an oil lamp. He also advocated the standard candle for luminosity measurement. More lastingly, he established the Royal Institution of Great Britain (1800), introducing Davy as director. He went to France in 1804 and settled in Paris, where he married Lavoisier's widow. The marriage was unhappy and ended after four years (Rumford is said to have suggested that Lavoisier was lucky to have been guillotined). Rumford himself appears to have been a disloyal and unappealing character, although at the end of his life he left most of his estate to the United States.