Michell studied at Cambridge University and became a fellow. In 1762 he was appointed Woodward Professor of Geology but left academic life to take up a post as rector at Thornhill, Yorkshire, in 1764.

Before his departure from Cambridge he published, in 1760, a fundamental paper, Conjectures Concerning the Cause, and Observations upon the Phenomena of Earthquakes. After the great Lisbon earthquake (1755) this was a fashionable subject. Michell assigned the cause of earthquakes to the force generated by high-pressure steam, produced when water suddenly met subterranean fires. He appreciated that such a force would generate waves in the Earth's crust and tried to estimate the velocity of these, giving a not unreasonable figure of 1200 miles per hour. Finally, Michell showed various means to determine the point of origin of the earthquake.

In 1790 he constructed a torsion balance to measure gravitational attraction and thus the mean density of the Earth. Michell was unable to use this before his death, but Henry Cavendish carried on his work, deriving a value for the density of the Earth in 1798.

Michell also made contributions to astronomy. In 1767 he published a paper on double stars, pointing out with originality and insight that there are far too many of them to result from a random scattering and therefore they must in many cases constitute a genuine binary system. He also devised a method for calculating the distance of the stars.