Magnetization of Martian crust has been modified by impact-induced shock waves and viscous decay since the cessation of the core dynamo of Mars at around 4 Gyr ago. Thermal evolution models of Mars suggest that the potentially magnetic layer was about 85 km thick during the active period of the core dynamo, assuming magnetite as the major magnetic carrier. The lower boundary of the magnetic layer has gradually decreased, by a total of about 30 km, through viscous decay of magnetization. The large impacts that created the giant basins Hellas, Argyre, and Isidis have almost completely demagnetized the crust beneath the basins. The shock wave pressure produced by impacts that created craters of diameters 300--1000 km is expected to significantly demagnetize the crust beneath the craters. However, except for a few craters, there is no signature of appreciable demagnetization. This implies that either the magnetic carriers have high coercivity and have resisted demagnetization, or magnetic source bodies are deep seated, or they have acquired magnetization after the intensive impact cratering period. An alternate possibility is that the scaling laws proposed for small craters do not apply to the large craters considered in this paper.