Cassini was born at the Paris Observatory. Admitted at the age of seventeen to membership of the French Academy of Sciences, he was elected in 1696 a fellow of the Royal Society of London, and became maître des comptes in 1706. Having succeeded to his father's position at the observatory in 1712, he measured in 1713 the arc of the meridian from Dunkirk to Perpignan, and published the results in a volume entitled Traité de la grandeur et de la figure de la terre (1720). His two separate calculations for a degree of meridian arc were 57,097 toises de Paris (111.282 km) and 57,061 toises (111.211 km), giving results for Earth's radius of 3,271,420 toises (6,375.998 km) and 3,269,297 toises (6,371.860 km), respectively.
He also wrote Eléments d'astronomie on proper motion (1740).
He died at Thury, near Clermont, France.
The asteroid 24102 Jacquescassini is named after him.
Jacques Cassini married Suzanne Françoise Charpentier de Charmois. Their second son was astronomer César-François Cassini de Thury, who was also known as Cassini III.