His research focuses on the dynamics of rings and small moons in the Solar System. Known for his persistence in planetary image analysis, Mark's early work with Voyager data led to the discoveries of Jupiter's faint, outer "gossamer" rings and Saturn's tiny ring-moon, Pan. Starting in 2003, his work with the Hubble Space Telescope led to the discoveries of "Mab" and "Cupid", small moons of Uranus now named after characters from Shakespeare's plays. His work also revealed two faint outer rings of dust encircling the planet. In 2011, Mark initiated a Hubble observing program focused on Pluto, which led to the discoveries of two tiny moons. Their names, "Kerberos" and "Styx", were selected through an international naming campaign. Mark also discovered the 14th known moon of Neptune. He is a co-investigator on NASA's Cassini mission to Saturn and its New Horizons mission to Pluto and beyond.

In addition to his research Mark manages the Ring-Moon Systems Node of NASA's Planetary Data System. The site provides public access to images and other data from NASA's Voyager, Galileo, Cassini and New Horizons missions, from the Hubble Space Telescope, and from a variety of Earth-based telescopes.