Pioneering chemist, agriculturalist, medical theorist, one of the most influential Scottish physicians of the eighteenth century, and a central figure of the Scottish Enlightenment. He was the son of William Cullen, a lawyer and factor to the Duke of Hamilton, and Elizabeth Roberton of Whistlebury. Educated first at the local grammar school and later at the University of Glasgow, he initially apprenticed to a surgeon-apothecary before spending several years as a ship's surgeon in the West Indies. Returning to Scotland in the early 1730s, he resumed his medical studies at Edinburgh and began to establish himself as a physician of growing reputation. In 1736 he settled in Hamilton, where he practised medicine for over a decade and developed a strong interest in chemistry, botany, and the improvement of agriculture.

His academic career began in earnest when he was appointed Lecturer in Chemistry and Medicine at the University of Glasgow in 1747. His lectures were immediately popular, and he became known for his clarity, originality, and ability to integrate chemistry with medical theory - an approach that was unusual for the time. It was during his Glasgow years that he conducted the experiments that laid the foundations of modern refrigeration. In 1748 he demonstrated the cooling effect produced by the rapid evaporation of volatile liquids under reduced pressure, a principle he later illustrated publicly in Edinburgh in 1756 by creating a small quantity of ice using a vacuum pump and ether. Although the process had no immediate commercial application, it represented the first documented demonstration of artificial refrigeration and remains one of his most significant scientific contributions.

In 1755, he moved to the University of Edinburgh, where he succeeded to the Chair of Chemistry and later to the Chair of the Institutes of Medicine. Edinburgh at this time was the leading medical school in Europe, and he quickly became one of its most celebrated professors. His lectures attracted students from across Britain, the American colonies, and continental Europe, and he played a decisive role in shaping the curriculum and intellectual character of the school. His medical writings, particularly his First Lines of the Practice of Physic, were widely read and translated, and his classification of diseases, though later superseded, was influential in the development of clinical teaching. His theories emphasised the role of the nervous system in disease and introduced the concept of "excitability", which influenced later medical thinkers including John Brown and the early physiologists of the nineteenth century.

Beyond medicine, he was deeply engaged with the wider intellectual life of the Enlightenment. He corresponded with leading figures such as Joseph Black, William Hunter, and Lord Kames, and he encouraged his students to pursue scientific inquiry across disciplines. His interest in agriculture led him to promote improved farming methods, soil analysis, and the application of chemistry to agricultural practice. He was also a founding member of the Royal Medical Society and an active participant in Edinburgh's learned societies. His influence extended through his pupils, among them many of the most distinguished physicians of the late eighteenth century, including William Withering, Benjamin Rush, and James Lind.

William Cullen retired from teaching in 1789 after more than forty years of academic service. He passed away in Edinburgh on the 5th February 1790, and was buried in Kirknewton Cemetery, West Lothian, where his grave remains preserved. He was survived by several children, including Lord Robert Cullen, later a Senator of the College of Justice, and Robina Millar, whose memorials stand alongside his. His legacy is that of a polymathic physician whose work helped transform medical education, whose refrigeration experiments anticipated one of the most important technologies of the modern age, and whose teaching shaped generations of practitioners across Britain, Europe, and America. In the history of Scottish medicine, he stands as one of its most formative and enduring figures.