We have studied the effect of magnetic disorder on the magnetization reversal process in thin Co/CoO-films. The antiferromagnetic CoO layer allows a reversible tuning of the magnetic disorder by simple temperature variation, which was experimentally verified by measuring the ferromagnetic resonance linewidth as a function of temperature. The Co/CoO-films exhibit a discontinuous magnetization reversal process for temperatures above a critical temperature Tc, whereas smooth magnetization loops occur for T<Tc. Our measurements establish the existence of a disorder-driven critical point in the non-equilibrium phase diagram. In addition, we find that the magnetization reversal curve exhibits an asymmetry, which switches at the critical point from a precursor mechanism for T Tc.