To this business he succeeded on his father's death in 1759. He went into partnership with John Fothergill and in consequence of the growth of their business removed his works in 1762 from Snowhill to what was then a tract of barren heath at Soho, 2 miles north of Birmingham. Here he undertook the manufacture of artistic objects in metal, as well as the reproduction of oil paintings by a mechanical process in which he was associated with Francis Eginton (1737-1805), who subsequently achieved a reputation as a worker in stained or enamelled glass. In this he was also encouraged by Robert Adam. Between 1762 and 1775 he established a strong reputation as a craftsmen; his works at Soho were widely known for excellent and artistic workmanship.
About 1767, Boulton, who was finding the need of improving the motive power for his machinery, made the acquaintance of James Watt, who on his side appreciated the advantages offered by the Soho works for the development of his steam-engine. In 1772 Watt's partner, Dr John Roebuck, got into financial difficulties, and Boulton, to whom he owed £1200, accepted the two-thirds share in Watt's patent held by him in satisfaction of the debt. Three years later Boulton and Watt formally entered into partnership, and it was mainly through the energy and self-sacrifice of the former, who devoted all the capital he possessed or could borrow to the enterprise, that the steam engine was at length made a commercial success.
It was also owing to Boulton that in 1775 an act of parliament was obtained extending the term of Watt's 1769 patent to 1799]. In 1800 the two partners retired from the business, which they handed over to their sons, Matthew Robinson Boulton and James Watt junior.
In 1788 Boulton turned his attention to coining machinery, and erected at Soho a complete plant with which he struck coins for the Sierra Leone and East India companies and for Russia, and in 1797 produced a new copper coinage for Great Britain. In 1797 he took out a patent in connection with raising water on the principle of the hydraulic ram. He died at Birmingham on August 18 1809.