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James Watt was born on 19th of January, 1736, in Greenock, a seaport on the Firth of Clyde. The fourth child by his father James a shipwright, ship owner and contractor, while his mother, Agnus Muirhead, came from a distinguished family and was well educated.

Watt attended school irregularly and instead was mostly schooled at home by his mother who taught him reading and his father writing and arithmatic. He exhibited great manual dexterity and an aptitude for mathematics, taking toys to pieces and rebuilding whilst improving the design. When he was 17, his mother died and his father's health had begun to fail. Watt travelled to London to study mathematical instrument-making for a year, then in August 1756 he returned to Glasgow. The University of Glasgow, by middsummer 1757 offered him the opportunity to set up a small workshop within the university. In 1763 Watt left his University rooms for his own premises and in 1764, Watt married his cousin Margaret Miller, with whom he had five children, two of whom lived to adulthood. She died in childbirth in 1772. In 1777 he married again, to Ann MacGregor, daughter of a Glasgow dye-maker, who survived him. She died in 1832.

Four years after opening his shop, Watt began to experiment with steam after his friend, Professor John Robison, called his attention to it. At this point Watt had still never seen an operating steam engine, but he tried constructing a model. In 1761/2 he tried some experiments using Denis Papin's digester. He independently discovered the importance of latent heat in understanding the engine, which, unknown to him, Black had famously discovered some years before. He learned that the University owned a model Newcomen engine, but it was in London for repairs. Watt got the university to have it returned, and he made the repairs in 1763. It too just barely worked, and after much experimentation he showed that about 80% of the heat of the steam was consumed in heating the cylinder, because the steam in it was condensed by an injected stream of cold water. His critical insight, to cause the steam to condense in a separate chamber apart from the piston, and to maintain the temperature of the cylinder at the same temperature as the injected steam, came finally in 1765 and he soon had a working model.

Now came a long struggle to produce a full-scale engine. This required more capital, some of which came from Black. More substantial backing came from John Roebuck, the founder of the celebrated Carron Iron Works, near Falkirk, with whom he now formed a partnership. But the principal difficulty was in machining the piston and cylinder. Iron workers of the day were more like blacksmiths than machinists, so the results left much to be desired. Much capital was spent in pursuing the ground-breaking patent, which in those days required an act of parliament. Strapped for resources, Watt was forced to take up employment as a surveyor for eight years. Roebuck went bankrupt, and Matthew Boulton, who owned the Soho foundry works near Birmingham, acquired his patent rights. Watt and Boulton formed a hugely successful partnership (Boulton & Watt), which lasted for the next twenty-five years.

Watt finally had access to some of the best iron workers in the world. The difficulty of the manufacture of a large cylinder with a tightly fitting piston was solved by John Wilkinson who had developed precision boring techniques for cannon making at Bersham, near Wrexham, North Wales. Finally, in 1776, the first engines were installed and working in commercial enterprises. These first engines were used for pumps and produced only reciprocating motion. Orders began to pour in and for the next five years Watt was very busy installing more engines, mostly in Cornwall for pumping water out of mines.

The field of application of the invention was greatly widened only after Boulton urged Watt to convert the reciprocating motion of the piston to produce rotational power for grinding, weaving and milling. Although a crank seemed the logical and obvious solution to the conversion Watt and Boulton were stymied by a patent for this, whose holder, James Pickard, and associates proposed to cross-license the external condensor. Watt adamantly opposed this and they circumvented the patent by their sun and planet gear in 1781.

Over the next six years, he made a number of other improvements and modifications to the steam engine. A double acting engine, in which the steam acted alternately on the two sides of the piston was one. A throttle valve to control the power of the engine, and a centrifugal governor to keep it from "running away" were very important. He described methods for working the steam expansively. A compound engine, which connected two or more engines was described. Two more patents were granted for these in 1781 and 1782. Numerous other improvements that made for easier manufacture and installation were continually implemented. One of these included the use of the steam indicator which produced an informative plot of the pressure in the cylinder against its volume, which he kept as a trade secret. Another important invention, one of which Watt was most proud of, was the Parallel motion / three-bar linkage which was especially important in double-acting engines as it produced the straight line motion required for the cylinder rod and pump, from the connected rocking beam, whose end moves in a circular arc. This was patented in 1784. These improvements taken together produced an engine which was up to five times as efficient in its use of fuel as the Newcomen engine.

Because of the danger of exploding boilers and the ongoing issues with leaks, Watt was opposed from the first to the use of high pressure steam--all of his engines used steam at very low pressure.

In 1794 the partners established Boulton and Watt to exclusively manufacture steam engines, and this became a large enterprise. By 1824 it had produced 1164 steam engines having a total nominal horsepower of about 26,000.[3] Boulton proved to be an excellent businessman, and both men eventually made fortunes.

Watt was an enthusiastic inventor, with a fertile imagination that sometimes got in the way of finishing his works, because he could always see "just one more improvement." He was skilled with his hands, and was also able to perform systematic scientific measurements that could quantify the improvements he made and produce a greater understanding of the phenomenon he was working with.

Watt was a gentleman, greatly respected by other prominent men of the Industrial Revolution. He was an important member of the Lunar Society, and was a much sought after conversationalist and companion, always interested in expanding his horizons. He was a rather poor businessman, and especially hated bargaining and negotiating terms with those who sought to utilize the steam engine. Until he retired, he was always much concerned about his financial affairs, and was something of a worrier. His personal relationships with his friends and partners were always congenial and long-lasting.

Watt retired in 1800, the same year that his fundamental patent and partnership with Boulton expired. The famous partnership was transferred to the men's sons, Matthew Boulton and James Watt Jr. William Murdoch was made a partner and the firm prospered.

Watt continued to invent other things before and during his semi-retirement. He invented a new method of measuring distances by telescope, a device for copying letters, improvements in the oil lamp, a steam mangle and a machine for copying sculptures.

He and his second wife travelled to France and Germany, and he purchased an estate in Wales, which he much improved.

He died in his home "Heathfield" in Handsworth, Staffordshire on 19 August 1819 at the age of 83.