After he left the school in 1761, he initially took part in the survey for the British coastal charts, and was then sent on a mission to Martinique in 1764 to take part in the construction of the Fort Bourbon under the orders of the lieutenant-colonel of Rochemore, as the French colony was insulated in the middle of the English and Spanish possessions following the Seven Years' War. Coulomb spent eight years directing the work, contracting tropical fever. He carried out several experiments on the resistance of masonries and the behaviour of the walls of escarpe (supportings), which were inspired by the ideas of Pieter van Musschenbroek on friction.
Upon his return, with the rank of Captain, he was employed at La Rochelle, the Isle of Aix and Cherbourg. He discovered an inverse relationship of the force between electric charges and the square of its distance, later named after him as Coulomb's law.
In 1781, he was stationed permanently at Paris. On the outbreak of the Revolution in 1789, he resigned his appointment as intendant des eaux et fontaines, and retired to a small estate which he possessed at Blois. He was recalled to Paris for a time in order to take part in the new determination of weights and measures, which had been decreed by the Revolutionary government. He was one of the first members of the National Institute; he was appointed inspector of public instruction in 1802. His health was already very feeble, and four years later he died in Paris, France.
In 1785 Coulomb presented his three reports on Electricity and Magnetism:
- Premier Mémoire sur l’Electricité et le Magnétisme . In this publication Coulomb describes “How to construct and use an electric balance (torsion balance) based on the property of the metal wires of having a reaction torsion force proportional to the torsion angle”. Coulomb also experimentally determined the law that explains how “two bodies electrified of the same kind of Electricity exert on each other”.
- Sécond Mémoire sur l’Electricité et le Magnétisme . In this publication Coulomb carries out the “determination according to which laws both the Magnetic and the Electric fluids act, either by repulsion or by attraction”.
- Troisième Mémoire sur l’Electricité et le Magnétisme . “On the quantity of Electricity that an isolated body loses in a certain time period , either by contact with less humid air, or in the supports more or less idio-electric”.
Four subsequent reports were published in the following years:
- Quatrième Mémoire "Where two principal properties of the electric fluid are demonstrated: first, that this fluid does not expand into any object according to a chemical affinity, or by an elective attraction, but that it divides itself between different objects brought into contact; second, that in conducting objects, the fluid, having achieved a state of stability, expands on the surface of the body and does not penetrate into the interior." (1786)
- Cinquième Mémoire "On the manner in which the electric fluid divides itself between conducting objects brought into contact, and the distribution of this fluid on the different parts of the surface of this object." (1787)
- Sixième Mémoire "Continuation of research into the distribution of the electric fluid between several conductors. Determination of electric density at different points on the surface of these bodies." (1788)
- Septième Mémoire. "On magnetism" (1789)
Coulomb explained the laws of attraction and repulsion between electric charges and magnetic poles, although he did not find any relationship between the two phenomena. He thought that the attraction and repulsion were due to different kinds of fluids.