Pierre-Simon Laplace's father expected him to make a career in the Church. At the age of 16, Laplace entered Caen University to study theology. However, during his two years at the University of Caen, Laplace discovered his mathematical talents and his love of the subject. Consequently, he left Caen without taking his degree, and went to Paris. He took with him a letter of introduction to d'Alembert. Although Laplace was only 19 years old when he arrived in Paris he quickly impressed d'Alembert, so d'Alembert got him the job of professor of mathematics at the Ecole Militaire.

He began producing a steady stream of remarkable mathematical papers. His first paper was on maxima and minima of curves where he improved on methods given by Lagrange. His next paper concerned difference equations. He quickly wrote papers on the integral calculus, mechanics, physical astronomy, and mathematical astronomy. In 1773, he was elected an adjoint in the Acadèmie des Sciences. By the time of his election he had read 13 papers to the Academy in less than 3 years.

Laplace's reputation steadily increased during the 1770's. The 1780's were the period in which Laplace produced the depth of results which have made him one of the most important and influential scientists that the world has seen. It does appear however that Laplace was not modest about his abilities and achievements, and he probably failed to recognise the effect of his attitude on his colleagues. Laplace let it be known widely that he considered himself the best mathematician in France. The effect on his colleagues was only mildly eased by the fact that Laplace was right.

In 1780, Laplace and the chemist Lavoisier showed respiration to be a form of combustion. This work with Lavoisier marked the beginning of a third important area of research for Laplace, namely his work on the theory of heat, which he worked on towards the end of his career.

In 1784 Laplace was appointed as examiner at the Royal Artillery Corps, and in this role in 1785, he examined and passed the 16 year-old Napoleon Bonaparte. He also served on many of the committees of the Académie des Sciences. Laplace was promoted to a senior position in the Académie des Sciences in 1785. Two years later Lagrange also came to Paris. The two great mathematical geniuses, despite a rivalry between them, each was to benefit greatly from the ideas flowing from the other.

Laplace was a member of the committee of the Académie des Sciences to standardise weights and measures in May 1790. This committee worked on the metric system and advocated a decimal base.

In 1795, the Ecole Normale was founded with the aim of training school teachers and Laplace taught courses there, including one on probability. The Ecole Normale survived for only 4 months, for the pupils, who were training to become school teachers, found the level of teaching well beyond them.

In 1795, the Bureau des Longitudes was founded with Lagrange and Laplace as the mathematicians among its founding members, and Laplace went on to lead the Bureau and the Paris Observatory. Laplace presented his famous nebular hypothesis in 1796, which viewed the solar system as originating from the contracting and cooling of a large, flattened, and slowly rotating cloud of incandescent gas. His work consisted of five books: the first was on the apparent motions of the celestial bodies, the motion of the sea, and also atmospheric refraction; the second was on the actual motion of the celestial bodies; the third was on force and momentum; the fourth was on the theory of universal gravitation and included an account of the motion of the sea and the shape of the Earth; the final book gave an historical account of astronomy and included his famous nebular hypothesis.

Laplace had already discovered the invariability of planetary mean motions. In 1786 he had proved that the eccentricities and inclinations of planetary orbits to each other always remain small, constant, and self-correcting. These and many other of his earlier results formed the basis for his great work, the Traité du Mécanique Céleste published in 5 volumes, the first 2 in 1799. The first volume is divided into 2 books, the first on general laws of equilibrium and motion of solids and also fluids, while the second book is on the law of universal gravitation and the motions of the centres of gravity of the bodies in the solar system. The second volume deals with mechanics applied to a study of the planets. In it Laplace included a study of the shape of the Earth which included a discussion of data obtained from several different expeditions, and Laplace applied his theory of errors to the results. Another topic studied here by Laplace was the theory of the tides. Laplace's equation appears here, but it was in fact known before the time of Laplace. The Legendre functions also appear here, and were known for many years as the Laplace coefficients.

Napoleon, made Laplace Minister of the Interior, but dismissed him after 6 weeks "because he brought the spirit of the infinitely small into the government". Laplace became Count of the Empire in 1806, and he was named a marquis in 1817.

The first edition of Laplace's Théorie Analytique des Probabilités was published in 1812, covering generating functions, approximations to various expressions occurring in probability theory, Laplace's definition of probability, Bayes's rule, least squares, Buffon's needle problem, inverse probability, and applications to mortality, life expectancy, the length of marriages, and legal matters.

His approach to physics, attempting to explain everything from the forces acting locally between molecules, influenced physics greatly. In 1805, he wrote a study of pressure and density, astronomical refraction, barometric pressure and the transmission of gravity based on this new philosophy of physics. Laplace continued to apply his ideas of physics to other problems such as capillary action, double refraction, the velocity of sound, the theory of heat, and elastic fluids, and he wrote papers on all these subjects.