The seismic events recently occurred all over the world and, in particular, in Europe have shown the high vulnerability of particular classes of buildings against the horizontal actions. The damage of structural masonry walls is one of the most widespread harming injuries and cause of loss of serviceability and seismic capacity for a building. The damage experienced by these masonry elements has brought to light the necessity to strengthen them with appropriate reinforcing systems in order to achieve an upgrading to the necessary seismic and energy dissipation capacity. Different strengthening systems have been proposed and studied during the last decades, with particular reference to the type of materials, system configuration with respect to the element to be strengthened, difficulties in the application and effectiveness of the reinforcement. Even though in the last years different studies have been carried out in this field, many issues regarding the methods for the evaluation of the actual behaviour of these techniques, and their effectiveness in the improvement of seismic behaviour of structural members to which they are applied, are still open. In the present study the structural behaviour of unreinforced masonry walls strengthened with composite grid reinforced mortar layers is studied. The characterization of the reinforcing system and the assessment of the overall increase of capacity of the strengthened masonry walls is performed. First of all, the study is focused on the investigation of the mechanical characteristics of the strengthening system in itself. In fact, the structural behaviour of an externally applied strengthening system for masonry walls is examined. The reinforcing technique considered in the present research is composed by mortar layers incorporating a FRP reinforcement in form of grid. The FRP reinforced mortar layers are externally applied to the wall surfaces in a symmetric fashion, and can also be connected to the wall by means of an adequate connection system. The mechanical behaviour of the reinforced mortar under tensile, compression and shear loading is assessed through laboratory tests and constitutive laws can be proposed to characterize the reinforced mortar mechanical behaviour. The experimental characterization of the presented system is followed by and validated through numerical modelling and simulation of its mechanical behaviour. In a second phase, the behaviour of masonry walls strengthened by means of the considered technique is studied. The in-plane shear behaviour is considered, in case of cyclic loading state. The performances of the global strengthened assemblage is thus examined with both experimental and numerical investigation criteria. Also, the overall ductility and energy dissipation capacity of the system, while subjected to horizontal in-plane actions, is studied. The actual mechanical behaviour of the proposed structural solution is investigated through an experimental program with prototypes. Furthermore, a finite element model is realized in order to replicate the structural features of the strengthened masonry wall. The finite element model can be also used for further validation of the panels performances.