Stephen Demainbray was born in London in 1710. His father died shortly after his birth, so he was brought up by his uncle, an officer in the English service, and page of honour to Queen Mary, who placed him at Westminster school. Whilst studying at Westminster, he boarded in the house of Dr. Desaguliers, who instructed him in mathematics and natural philosophy. He married at 17, before finishing school, before going to Leyden to continue his studies. In 1740 he began to read lectures in experimental philosophy at Edinburgh.

In 1746, Demainbray published his discovery of the effects of electricity on the growth of vegetables. In 1749 he began giving lectures, and in 1753 he was invited to read a course of lectures to the Prince of Wales (later King George III) and the Duke of York. He then gave private courses to other branches of the Royal family; and when the future Queen Charlotte arrived in England, he instructed her in experimental philosophy and natural history. In 1768 he was appointed aftronomer to King George III's new observatory at Richmond, and adjusted the instruments there in time to observe the transit of Venus, which happened the following year.

Dr. Demainbray died on 20 February 1782 in Northolt, London.