Crustal magnetic fields influence a range of plasma processes at Mars, guiding the flow of energy from the solar wind into the planet’s atmosphere at some locations while shielding it at others. In this study we investigate how these crustal fields vary with changes in the direction of the incoming interplanetary magnetic field (IMF). Using spacecraft measurements of electron flux and magnetic field, we analyze magnetic topology throughout the Martian ionosphere for different IMF configurations. We find that the topology of crustal field cusp regions is dependent on IMF direction, and that cusps transition between open and closed topology regularly as they rotate through the nightside of Mars. Finally, we determine that cusps often become topologically closed due to reconnection with open magnetic fields in the Martian magnetotail, resulting in large day-to-night closed loops that could represent a source of energy input into nightside crustal field regions.