A young magnetically active Sun, with enhanced ionizing radiations and an elevated solar wind, might have contributed to erosion of the primordial atmosphere of Mars (which is particularly vulnerable to dissociative recombination and sputtering by solar wind pickup ions). Spacecraft and ground based observations of solar-type dwarfs in young galactic clusters have yielded a unified view of the early evolution and subsequent systematic decline of magnetic activity with age. Rotational braking by the coronal wind ultimately quenches the spin-catalyzed dynamo at the heart of stellar magnetism. Decay of the 106--107 K corona is much faster than the 104 K chromosphere, but XUV emissions of both can be predicted reliably, and photoionization of key planetary atmospheric constituents can be modeled. The early Martian atmosphere (age ~1 Gyr) probably was subjected to photoionization rates ~5 times contemporary peak values (sunspot maximum), possibly more if the ages of galactic clusters have been underestimated.¿ 1997 American Geophysical Union