Impact identification is a major concern in structural health monitoring. It consists in localizing the impact point and in reconstructing the applied load history from indirect measurements. However, it is well-known that this double inverse problem is ill-posed. In this paper, a single-sensor approach for quickly localizing a Dirac-like impact and for estimating its intensity from vibration measurements is proposed. It is proved that the relative proportions of specific modal ponderations are a signature of impact location. Rules are provided to select the appropriate vibration modes in the analysis and to position the sensor. The identification process is robust if the vibration modes kept in the analysis are weakly damped and if their natural frequencies are well separated. Experiments are conducted on a metallic plate to validate this single-sensor approach on a 2D structure. The success rate for localizing impacted cells is 100% when appropriate method parameters are selected.