Most historical buildings and structures in Oman were built using unreinforced stone masonry. These structures have deteriorated due to ageing of materials, environmental degradation and lack of maintenance as these structures have negligible resistance to shearstresses. This research presents a study of the physical, chemical and mechanical properties of local building materials and the results of an experimental study on the in-plane shear effectiveness of a modern strengthening technique applied to existing masonry walls. The technique consists of the application, on one and two faces of the walls of a textile reinforced mortar (TRM). Shear loading tests of full-scale masonry samples (1000 mm width, 1000 mm height and 350 mm depth) were carried out considering one unreinforced specimen and six different cases of reinforced specimens. The performance of unreinforced and reinforced specimens was analyzed and compared. It was shown that strengthened specimens can resist in-plane shear stresses almost 1.5 to 2.1 times greater than those of unreinforced specimen; moreover, they can provide ductility rather than sudden failure, due to the presence of the fiberglass and basalt mesh, which restricts the opening of cracks.