His earliest education was at home in the Paris Observatory, an excellent place for a boy with enthusiastic interests in science. Later, he studied at the Collège Mazarin in Paris. Pierre Varignon, who had been appointed professor of mathematics in 1688, supervised Cassini's thesis on optics. In August 1691, at the age of fourteen, Cassini defended his thesis.
Three years later, he was admitted to the Académie des Sciences and began to work with his father on scientific projects. In 1635, he traveled with his father in Italy and they made numerous geodesic observations. They also visited Bologna where they repaired the gnomon at the Church of San Petronio which Cassini's father had designed nearly thirty years before. The younger Cassini continued his travels, visiting Flanders and around 1698, traveling to England where he Newton, Flamsteed, and Halley and was elected to the Royal Society of London.
In 1700, father and son worked together to attempt to measure the meridian from Paris to Perpignan, which is 13 km west of the Mediterranean coast. Unfortunately, the results they obtained suggested (incorrectly) that the Earth was elongated at the poles.
Astronomy was not Jacques' only interest. He had some legal training, but did not become a lawyer. In 1706, he was appointed to the Chambre des Comptes in 1706, a financial court relating to the King's accounts. While he distinguished himself for his honesty, he gained the reputation of being indecisive. His legal career continued as he was appointed an advocate in the Court of Justice in 1716 and later in 1722 became a Councillor of State.
Meanwhile, his importance to science continued to grow as he continued to work with the Académie des Sciences. Around 1709, as his father's health deteriorated, Jacques began to assume his responsibilities and upon the death of his father in 1712, he succeeded him as the director of the Paris Observatory.
During the intervening years, in 1710 he married Suzanne-Françoise Charpentier from Charmois. During their years together, they had three sons, Dominique-Jean, César-François who was later known as Cassini III or Cassini de Thury, and Dominique-Joseph, and two daughters Suzanne-Françoise and Elisabeth-Germaine. The eldest son Dominique-Jean followed his father into the Chambre des Comptes while the youngest son followed a military career.
Jacques Cassini continued his surveying work, taking on the project to measure the Paris meridian north to Dunkerque in 1718. Just as he had accompanied his father when he was younger, on this trip he was accompanied by his own son César-François Cassini. Afterward, he published the results, which again supported his incorrect theory of elongation at the poles.
In his work with astronomy, Cassini was well published especially in the journals of the Académie des Sciences as well as writing two major treatises; "Elements of Astronomy" and "Astronomical Tables of the Sun, Moon, Planets, Fixed Stars, and Satellites of Jupiter and Saturn." He is especially known for study of the moons of Jupiter and Saturn and his study of the structure of Saturn's rings. While he began slowing down with his own work around 1740, for several years afterward, he assisted his son Cassini de Thury (César-François).
While visiting his Château at Thury, he was involved in a accident. His carriage overturned, seriously injuring Jacques. Cassini died of his injuries on April 18, 1756 and he was buried in the Church at Thury.