Rainfall-induced slope failures are a major cause of slope failure, with incidents likely to increase with the predicted escalation of extreme rainfall events. Traditional numerical methods such as the Finite Element Method (FEM) are often restricted in their applicability to small deformation analyses. Therefore, an incomplete description of the failure mechanism is given, in which the failure consequences, or evolving deformations and progressive failures, are ignored. A one-point, two-phase Material Point Method (MPM) formulation is proposed to consider the influence of rainfall on slope failure. Due to the characteristics of MPM in capturing the large deformations, a complete failure process, from initiation to failure, of a slope subjected to rainfall infiltration is presented. The soil behaviour is described by a Mohr–Coulomb strain softening model based on Bishop’s stress. A retrogressive failure mode with an initial shallow instability is observed during the slope failure. In contrast with a comparative total stress analysis, i.e. without consideration of pore pressures, the two-phase analysis shows that the rainfall-affected slope is initially stable, until the suction stresses are reduced leading to a superficial failure mode, which in turn leads to a complete slope failure.