We find Mg illuminated in vacuum with picosecond UV pulses to be a robust and useful photocathode even at roughing pressures. Photocurrent densities as great as 180 A/cm2 occur at moderate collecting field strengths E of 0.5 to 2 kV/cm. In pulses briefer than the electron transit time across a diode, space charge causes the collected charge to increase linearly with anode voltage. The observed Mg quantum efficiency of 2.5 x 10-6 electrons per 300 nm photon was doubled by "red assist" when intense 600 nm pulses were simultaneously present. We give a criterion for Emin for complete charge collection from trains of picosecond optical pulses.