An energy-balance model is often used to analyze impact dynamics for composite structures. However this model tends to overestimate the peak impact load after the onset of damage since it does not account for damage initiation and propagation. In this paper, the energy-balance model is coupled with the law of conservation of momentum to extend its validity beyond the elastic response regime for a composite sandwich structure subjected to low-velocity impact. Closed-form solutions were derived for the plate's elastic structural stiffness and the critical load at the onset of damage. The critical load was theoretically predictable by accounting for the elastic energies absorbed by the plate up to core failure. Impact test results also showed that the relative loss of the plate's transverse stiffness after damage was directly related to the energy absorbed by the plate, which could be calculated given the damage initiation energy. The stiffnesses and the critical load were then used in the modified energy-balance model to predict transient load and deflection histories. Predicted results were comparable with test data, in terms of critical and peak loads, as well as the overall behaviour. This impact model is an efficient design tool which can complement detailed FE simulations.