At the age of fourteen he was acquainted with Greek, Latin, French, Italian, Hebrew, Chaldean, Syriac, Samaritan, Arabic, Persian, Turkish and Amharic. Beginning to study medicine in London in 1792, he removed to Edinburgh in 1794, and a year later went to Göttingen, where he obtained the degree of doctor of physic in 1796. In 1797 he entered Emmanuel College, Cambridge. In the same year the death of his grand-uncle, Richard Brocklesby, made him financially independent, and in 1799 he established himself as a physician in Welbeck Street, London.
Appointed in 1801 professor of physics at the Royal Institution, in two years he delivered ninety-one lectures. These lectures, printed in 1807 (Course of Lectures on Natural Philosophy), contain a remarkable number of anticipations of later theories. He resigned his professorship in 1803, fearing that its duties would interfere with his medical practice.
In the previous year he was appointed foreign secretary of the Royal Society, of which he had been elected a fellow in 1794. In 1811 he became physician to St George's Hospital, and in 1814 he served on a committee appointed to consider the dangers involved by the general introduction of gas into London. In 1816 he was secretary of a commission charged with ascertaining the length of the seconds pendulum, and in 1818 he became secretary to the Board of Longitude and superintendent of the Nautical Almanac.
A few years before his death he became interested in life assurance, and in 1827 he was chosen one of the eight foreign associates of the French Academy of Sciences. He died in London on May 10, 1829.
Young is perhaps best known for his work in physical optics, as the author of a remarkable series of researches which did much to establish the wave theory of light, and as the discoverer of the interference of light.
He has also been called the founder of physiological optics. In 1793 he explained the mode in which the eye accommodates itself to vision at different distances as depending on change of the curvature of the crystalline lens; in 1801 he described the defect known as astigmatism; and in his Lectures he put forward the hypothesis, afterwards developed by Hermann von Helmholtz, that colour perception depends on the presence in the retina of three kinds of nerve fibres which respond respectively to red, green and violet light. In physiology he made an important contribution to haemadynamics in the Croonian lecture for 1808 on the "Functions of the Heart and Arteries," and his medical writings included An Introduction to Medical Literature, including a System of Practical Nosology (1813) and A Practical and Historical Treatise on Consumptive Diseases (1815).
In another field of research, he was one of the first successful workers at the decipherment of Egyptian hieroglyphics; by 1814 he had completely translated the enchorial (demotic) text of the Rosetta Stone, and a few years later had made considerable progress towards an understanding of the hieroglyphic alphabet. In 1823 he published an Account of the Recent Discoveries in Hieroglyphic Literature and Egyptian Antiquities. Some of his conclusions appeared in the famous article of Egypt which in 1818 he wrote for the Encyclopædia Britannica.