Cassini received his secondary education at the Collège du Plessis in Paris and at the Oratorian collège at Juilly. He then studied under the physicist Nollel, the mathematician Antoine Mauduit, and the astronomers Giovanni Maraldi and J. B. Chappe d’Auteroche.
In 1768, on a cruise in the Atlantic, Cassini was in charge of continuing the attempts to test a marine chronometer of Pierre Le Roy. Fleeted adjoint by the Académie des Sciences on 23 July 1770 (he became associé in 1785), he was put in charge of the publication of Chappe’s Voyage en Californie. Assured from 1771 of succession to the directorship of the Paris Observatory, a post created for his father, he gradually assumed its responsibilities before being officially appointed in 1784, upon the death of Cassini III.
In 1784 Cassini persuaded Louis XVI to agree to the restoration and reorganization of the observatory, a project, however, that he was able to realize only partially. He occupied himself with the completion of the great map of France undertaken by his father and in 1787, with A. M. Legendre and Méchain, participated in the geodesic operations joining the Paris and Greenwich meridians.
From March 1793 he opposed the reforms that the authorities wanted to introduce at the observatory, and he attempted to maintain his former authority. However, after bitter polemics, he gave up his duties on 6 September 1793 and a few weeks later left the Paris Observatory, which for 120 years had practically been the property of his family. Denounced by the revolutionary committee of Beauvais, Cassini was arrested in Paris on 14 February 1794 and imprisoned. On 5 August 1794, he retired to the family château of Thury, in the Oise. He participated in local affairs as a member of the board of examiners of the primary schools and of the École Centrale de l’Oise but declined his nomination to the Bureau des Longitudes at the end of 1795 and to the astronomy section of the new Institut National in January 1796.
A few years later Cassini attempted to resume his career. He accepted election as associé of the experimental physics section on 24 April 1798 and then as a member of the astronomy section of the Institute on 24 July 1799, but he strove in vain to secure renomination to the Bureau des Longitudes. Renouncing further pursuit of his scientific work, he assumed the presidency of the Conseil Général de l’Oise from 1800 to 1818. Pensioned and decorated by Napoleon and Louis XVIII, Cassini devoted himself to local politics and to polemical writings aimed at combating liberal ideas, defending his family’s scientific prestige, and justifying his attitude.
Cassini died on October 18, 1845, in Thury, France.
Cassini accepted some political duties at the beginning of the Revolution, and directed the execution of a portion of the new administrative maps and participated for several months, in 1791, in the work of the commission of the Academy responsible for preparing a new metrological system. He was firmly attached to the monarchy, however, and, little by little, adopted an attitude of hostile reserve toward the Revolution.
In 1773 Cassini married Claude-Marie-Louise de la Myre-Mory, who died in 1791, leaving him with five young children: Cécile, Angélique, Aline, Alexis, and Alexandre Henri Gabriel, later a jurist and botanist, with whom the French branch of the Cassini family died out.