This paper reports and analyses the results of a series of mechanical tests carried out on historic walls reinforced with an innovative technique by means of jacketing with GFRP (Glass Fiber Reinforced Plastics) grid inserted into an inorganic matrix made of cement based mortar in order to increase the lateral load-carrying and deformation capacity of unreinforced masonry (URM) walls. The basic construction material of analysed historic buildings are masonry assemblage of barely cut stones of different size and brickwork cemented by weak lime/sand mortar. This heterogeneous masonry typologies constitute a kind of low-strength masonry especially to horizontal seismic actions. Shear tests were carried out in-situ on panels cut from two different historic buildings in Italy: one in double-leaf rough hewn rubble stone masonry in Umbria and the second in the city of L'Aquila. Two widely-known test methods have been used for the experimental work: the diagonal tension test and the shear-compression test. The test results enabled the determination of the shear strength and shear modulus of the masonry wall panels before and after the application of the reinforcement. The panels strengthened with the GFRP exhibited a significant improvement in lateral load-carrying capacity of up to 1060% when compared to the control unreinforced panels.