Refrigerators for space and other applications working around 100 mK require lightweight components with good thermal properties. We have measured the thermal properties of high-purity (99.95%) magnesium, which is five times lighter than copper, over the 60-150 mK range and found that it is well-behaved down to these temperatures. Both conductivity and heat capacity are in good agreement with extrapolations from measurements at higher temperatures. The heat capacity per unit volume is about the same as copper and the thermal conductivity about 2.7 times lower than copper of similar residual resistivity ratio, as expected from magnesium's higher room-temperature resistivity.