This study evaluates the use of a composite material composed of multiaxial hybrid glass-fibers and polypropylene fabric coated in a hydraulic lime-based mortar (referred to as textile-reinforced mortar) to improve the in-plane shear strength and displacement capacity of solid clay-brick and hollow clay-block masonry walls with the aim of defining a comprehensive test protocol. Design parameters of the adopted strengthening system are investigated, and both single-sided and double-sided configurations for application on wall surfaces are considered. The influence of the presence of an external thermal insulation layer on walls’ in-plane performance is also considered. The first stage characterizes the materials, their bonding properties, and the initial shear strength of the masonry bed-joints. The second stage investigates the in-plane behavior of strengthened masonry wall panels using diagonal compression testing. No premature debonding is observed, even with no mechanical anchorage between the masonry substrate and the strengthening layer. Out-of-plane deformation due to structural asymmetry is observed in the single-sided strengthening configuration.