Using a pump–probe technique, the reactions of Mg(4 1S0 and 3 1D2) with H2 have been measured to yield similar rotational distributions of MgH(v=0 and 1) as that obtained for the reaction of the Mg(3 1P1) state with H2. A series of measurements is conducted to clarify that the reactions are initiated directly by these higher states, rather than occurring from the lower 3 1P1 state following radiative and collisional relaxation. The reactivity of the Mg 4 1S0 state with H2 is found to be comparable to that of the 3 1P1 state, but about three times larger than that of the 3 1D2 state. The Mg(4 1S0, 3 1D2)–H2 reactions proceed via a harpoon-type process, and are closely associated with the Mg(3 1P1)–H2 reaction coordinate through evolution of a series of surface crossings. To support our suggestion that the harpoon mechanism is involved, the cross sections of collisional deactivation by H2 for various excited states are measured. The ratios of cross sections observed for the 3 1P1, 4 1S0, and 5 1S0 state, equal to 1:2.85:4.3, are consistent with the calculated prediction of 1:2.62:4.24. The calculated cross sections are based on a simple hard sphere model with effective radii evaluated differently. Here, the effective radii for the higher states are determined from the crossing of ionic and covalent curves, while the Mg(3 1P1)–H2 radius is estimated from the nonadiabatic crossing between the reactive 1 1B2 state and the ground state. Consistency between observation and prediction confirms that the harpoon mechanism proposed in this work is plausible.