Cullen began his medical career by undertaking an apprenticeship with John Paisley, a practitioner in Glasgow who also work as the librarian to the Faculty of Physicians and Surgeons of Glasgow.

In 1729 he moved to London to take up a surgical post on a merchant ship travelling between London and the West Indies. After some time studying at the University of Edinburgh Medical School, as well as an apprenticeship with William Hunter, Cullen graduated MD from the University of Glasgow in 1740.

He began lecturing in chemistry at the University of Glasgow in 1747 and later lectured in materia medica and botany. Cullen was then appointed Professor of Medicine at the university in 1751. He moved to Edinburgh in 1755 and continued the rest of his career there, holding several positions at the University of Edinburgh and the Edinburgh Royal Infirmary.

Cullen was President of the Faculty of Physicians and Surgeons of Glasgow from 1747 to 1749. ]]>