We have developed a tunable intense narrow-band 285 nm light source based on frequency doubling of 570 nm light in BBO. At input powers of 840 mW (including 130 mW used for locking purposes) we generate 99 mW UV radiation with an intensity profile suitable for laser-cooling experiments. The light is used for laser cooling of neutral magnesium atoms in a magneto-optical trap (MOT). We capture about 5×106 atoms directly from a thermal beam and find that the major loss mechanism of the magnesium MOT is a near-resonant two-photon ionization process.