Using first-principles calculations, we have systematically investigated the magnetic properties of Mg-doped AlN (1 0 1 0) and (0 0 0 1) surfaces. Both the polar and non-polar surfaces are found to be magnetic and the magnetic moments are mainly due to spin polarized 2p orbitals of surface N atoms surrounding Mg. The splitting of energy levels in both cases favours charge hopping between the minority spin states of N 2p orbitals, which leads to a stable ferromagnetic ground state. However, the range of magnetic coupling and the stability of the ferromagnetic state differ between the polar and non-polar surfaces and are dependent on the nature of localization of the defect states. The ferromagnetic state in a Mg-doped reconstructed (0 0 0 1) surface is more stable than in a Mg-doped AlN (1 0 1 0) surface.