The strengthening method characterized by the application of composite grid reinforced mortar layers onto the surface of masonry walls is a quite new technique and, thus, still requires deep experimental and numerical investigation. This strengthening technique can be very effective to increase both the load-carrying and deformation capacity of unreinforced masonry walls subjected to cyclic loads. Moreover, it combines the advantages of both traditional and innovative types of intervention, since the composite grid is a high performance material and the mortar is a compatible material with respect to the masonry substrate. In the present paper the experimental results of in-plane shear tests on masonry walls subject to cyclic loads and externally strengthened with composite grid are synthetized. Then, the experimental results are compared with the numerical ones given by a Finite Element model. The FE model has been also used for carrying out several parametric analyses aimed at evidencing the effect of the reinforcement on both strength and displacement capacity of the wall. Two types of masonry, three values of shape factor (1.0, 1.3, and 2.0), different values of the compressive strength of the mortar layer and of the axial stiffness of the composite grid were investigated in the analysis. Finally, the effectiveness of the strengthening with mortar layers reinforced with a traditional steel grid has been investigated.