Grandson of a mathematician and son of a notable Scot of Greenock, it is of delicate health. In 1754, family difficulties lead it to work in London, then it returns to Glasgow, where it is charged, at the university, of the maintenance and the repair of the apparatuses of physics.

It become acquainted with Joseph Black there, chemistry teacher, who has just discovered the latent heat of vaporization of water.

In 1763, James Watt must precisely repair a steam engine of the atmospheric type; it realizes that the vapor consumption is very important, and which, in this kind of machine, the cooling of the cylinder which accompanies the condensation of the vapor is harmful with the output. He then imagines the separated condenser, to which he adds a pump to reinject water in the boiler. He takes a patent in 1769, and joins Matthiew Boulton, which has a manufacture close to Birmingham.

In 1781, in order to circumvent the rights of a patent protecting the crank-connecting rod system, he imagines a device with epicyclic gear transforming a reciprocating motion into a continuous rotation movement. He adds a wheel of inertia as well as the speed regulator to balls.

In 1782, it deposits a patent for the principle of the double-effect machine (the vapor acts alternatively on the two faces of the piston) and use of the relaxation (the vapor is allowed during a fraction of the travel of the piston and slackens then). He imagines a system of connection to rods articulated between the stem of piston and the beam, known under the name of parallelogram of Watt (1784).

The steam engines which leave the workshops Boulton and Watt as from 1784 (500 machines built until 1800) will not be supplanted during fifty years. It will be, in Great Britain initially, one of the causes of the industrial expansion of the XIXe century. The contribution of Watt, in the field of sciences and technology, is considerable, even if its step raised more than one logical research of solutions to a problem that application of scientific data.

James Watt was allowed with the Academy of Science in 1814. Its name was given to the unit of power (symbol W) usually used nowadays.