William Reynolds's education included some time studying with the physician and chemist Joseph Black. Reynolds during his life maintained his interest in many branches of science, including chemistry, geology and mineralogy, and he had a laboratory at his home in Ketley. He was interested in the application of science in industry. With Abraham Darby he built under licence several Boulton and Watt steam engines, an early type of steam engine, for the works.
Reynolds constructed three tub boat canals, which brought coal and iron ore a short distance from local mines to the ironworks. Built from 1787 to 1788, they were the Ketley Canal, the Wombridge Canal and the Shropshire Canal.They included an invention of Reynolds, an inclined plane: this was a method of raising canal boats from one level to another.
During this period, the reputation of Coalbrookdale encouraged visiting engineers who tried new ideas.
Thomas Telford knew the Coalbrookdale company since he had become Surveyor of Public Works for Shropshire in 1787. Reynolds constructed Longdon-on-Tern Aqueduct for Telford, a cast-iron aqueduct, assembled in 1796, carrying the Shrewsbury Canal across the River Tern at Longdon-on-Tern. (Telford, encouraged by its success, used cast iron for the Pontcysyllte Aqueduct).
In January 1802 Richard Trevithick came to Coalbrookdale, where boiler plates for steam engines had been made for some time. He built and tested a stationary engine which operated with an unprecedentedly high boiler pressure of 145 psi. Trevithick, who had built a road locomotive in Cornwall (the Puffing Devil), then worked, probably with Reynolds, to build a locomotive at Coalbrookdale that ran on rails; few details are known about this experiment, which is thought to be the first occasion of a locomotive on rails.
Around 1789 Richard Reynolds passed his shares in the Ketley ironworks to William and William's half-brother Joseph Reynolds (1768–1859). In 1796 the interests of the Darby and Reynolds families, having become complicated, were separated, the Ketley ironworks belonging to the Reynolds family, and Coalbrookdale to the Darbys.