[1] He was born in Bloomsbury, London the eldest son of George Hadley of Osidge, East Barnet, Hertfordshire and his wife Katherine FitzJames.

He had married Elizabeth, daughter of Thomas Hodges, FRS (former Attorney General of Barbados) and had one child, a son and heir John, born in 1738.

In 1730 Hadley invented the reflecting octant, which could be used to measure the altitude of the sun or other celestial objects above the horizon at sea.

Hadley also developed ways to make precision aspheric and parabolic objective mirrors for reflecting telescopes.

[5] This Newtonian, with a 6-inch-diameter (150 mm) primary mirror, compared favorably with the large aerial refracting telescopes of the day.