The use of new composite materials for reinforcement of heritage masonry structures, especially in seismic prone areas, is of interest structural engineers and conservators. However, the need to increase the structural performance of masonry structures is often in contrast with the principles of conservation in terms of reversibility, limited visual impact, compatibility of new materials with masonry. With the aim at striking a balance between structural safety and heritage protection, this paper investigates strengthening stone and brickwork masonry walls using glass-fiber reinforced polymer (GFRP) meshes embedded into a coating of lime or cement mortar. An experimental research program was undertaken in the laboratory on large-scale wall panels. Both clay brick and stone work specimens were tested, with and without strengthening. Single-sided and double-sided strengthenings were considered, as it is often not practicable to apply the reinforcement to both sides of a wall. Static tests were carried out on twelve masonry panels, under in-plane diagonal shear loading. The mechanisms by which load was carried were observed, varying from the initial, uncracked state, to the final, fully cracked state. The results demonstrate that a significant increase of the in-plane shear capacity of masonry can be achieved by using the proposed retrofitting technique. The experimental data were used to assess the effectiveness of the strengthening, and a finite element (FE) numerical model is discussed and calibrated against experimental results. The FE model was used to investigate further aspects of the reinforced masonry under shear-loading.