Bailly began his study of Halley's Comet in 1759. One year later he established an observatory where he could undertake observations of Jupiter's satellites. He was elected to the Académie des Sciences in 1763. His major works include Essai sur la théorie des satellites de Jupiter (1766; “Essay on the Theory of Jupiter's Satellites”) and Mémoires sur les inégalités de la lumière des satellites de Jupiter (1771; “Memoirs on the Uneven Illumination of Jupiter's Satellites”).

The French Revolution interrupted his studies. Elected deputy from Paris to the States General, he was chosen president of the third estate on May 5, 1789, and led the famous proceedings in the Tennis Court on June 20. He was proclaimed the first mayor of Paris on July 15, 1789. He was reelected mayor in August 1790 but lost popularity, particularly after his order to the national guard to disperse a riotous crowd led to the massacre of the Champ de Mars on July 17, 1791. Bailly retired on Nov. 16, 1791, and went to Nantes in July 1792, where he composed Mémoires d'un témoin de la Révolution (“Memoirs of a Witness of the Revolution”), an incomplete narrative of the extraordinary events of his public life. Late in 1793 Bailly went to Melun to join his friend Pierre-Simon Laplace, but was recognized, arrested, and taken before the revolutionary tribunal at Paris on November 10. He was subsequently guillotined.