Joule, the son of a brewer from Salford, received little formal education, was never appointed to an academic post, and remained a brewer all his life. He began work in a private laboratory that his father built near to the brewery.

His first major research was concerned with determining the quantity of heat produced by an electric current and, in 1840, Joule discovered a simple law connecting the current and resistance with the heat generated. For the next few years he carried out a series of experiments in which he investigated the conversion of electrical and mechanical work into heat. In 1849 he read his paper On the Mechanical Equivalent of Heat to the Royal Society. Joule's work (unlike that of Julius Mayer) was instantly recognized.

In 1848 Joule published a paper on the kinetic theory of gases, in which he estimated the speed of gas molecules. From 1852 he worked with William Thomson (later Lord Kelvin) on experiments on thermodynamics. Their best known result is the Joule–Kelvin effect – the effect in which an expanding gas, under certain conditions, is cooled by the expansion.