He made careful observations of Mars at every opposition starting in 1672 (according to Sheenan 1996), with particularly good results from the perihelic oppositions 1704 and 1719. He determined the rotation period of Mars, in particular that the same region returns to be visible after 36 rotations of Mars (or 37 rotations of Earth), and thus concluded on a rotation period of about 24h 39m, one minute less than his uncle G.D. Cassini had determined (the actual value is about 24h 37m). On Mars' surface, he discovered "Band-like Structures" (actually Mare Sirenum and Mare Tyrrhenum abledo features) as well as the "Hourglass Sea" (Syrtis Major), and studied the properties and changes of Mars' polar caps.
Moreover, he observed Jupiter's moons, observed comets 1699 I (C/1699 D1), 1702 (C/1702 H1), 1706 (C/1706 F1), 1707 (1707 W1), 1723 (C/1723 T1) and 1729 (C/1729 P1), and calculated various cometary orbits. He recognized that the corona visible in total solar eclipses belongs to the Sun and not to the Moon. In 1704, he discovered the variable star R Hydrae, a Mira type variable. In 1704 and 1705, he created a catalog of star position, in an early futile attempt to obtain stellar parallaxes.
J.P. Maraldi died on December 1, 1729 in Paris at age 64.
He was honored, together with his nephew, Jean-Dominique Maraldi (Maraldi II), by the naming of Moon crater Maraldi (19.4N, 34.9E, 39 km diameter, in 1935), and without explicitely mentioning his nephew, Mars crater Maraldi (62.0S, 32.0 W, 124km diameter, in 1973).