Using the multi-configuration Dirac–Fock self-consistent field method and the relativistic configuration interaction method, the calculations are carried out for the fine structure (FS) energy levels of Mg I, Al II, Si III, S V excited states (3s3d 1,3D, 3s4d 1,3D, 3s4s 1,3S, 3s3p 1,3P and 3s4p 1,3P). Breit interactions and quantum electrodynamics corrections are added as corrections. For Mg and Mg-like ions studied here, electron correlations are treated adequately, intravalence electron correlations in particular. The calculation results are found to be in good agreement with the experimental data and compare favourably with other theoretical results. By means of precise computational methods, we elucidate four competitive mechanisms which affect the interesting FS splitting orderings in the magnesium isoelectronic sequence.