Jean Picard studied at the Jesuit college at La Flèche. He worked with Gassendi for some time and helped him with observations of a solar eclipse on 21 August 1645. He received an M.A. from the University of Paris is 1650.
In 1655 Picard became professor of astronomy at the Collège de France in Paris. He became a member of the Académie Royale des Sciences in 1666, just after its foundation, and from this time on devoted himself to working for the Académie.
Picard devised a micrometer to measure the diameters of celestial objects such as the Sun, Moon and planets. In 1667 Picard added a telescope to the quadrant making it much more useful in observations. He greatly increased the accuracy of measurements of the Earth, using Snell's method of triangulation. He measured the length of the arc of the meridian; the measurements appear in Mesure de la Terre (1671). This data was used by Newton in his gravitational theory.
In the same year Picard went to Tycho Brahe's observatory at Hven Island in Sweden so that its location could be determined accurately and so Tycho observations could be directly compared with others. In 1673 Picard moved to the Paris Observatory where he collaborated with Cassini, Römer and, slightly later, with La Hire.
Picard was one of the first to apply scientific methods to the making of maps. He produced a map of the Paris region, then went on to join a project to map France. Among his other skills were hydraulics, a topic on which he wrote but one where he put his skills into practice; he solved the problem of supplying the fountains at Versailles with water.
Picard corresponded with many of the leading scientists of his time including Bartholin, Hevelius, Hudde and Huygens.