The mechanical behavior of Natural Textile Reinforced Mortars (NTRMs), made with flax textile and sustainable mortar matrix (Portland cement + metakaolin + fly ash + lime), are investigated in this paper. Both treated and untreated samples of flax textile are deeply characterized, including all the individual components (i.e., yarn, thread, and fabric). Then, flax fabric-TRMs are used for strengthening brick masonry walls, with and without the addition of the mortar matrix. As a result, treated flax fabric TRMs demonstrate superior mechanical performances and reduced crack openings compared to those untreated. Furthermore, walls strengthened with treated flax fabric layers exhibit improved strength and fracture toughness, unlike those with untreated fabric. Thus, if biomaterials are properly treated, they can effectively reinforce existing masonry structures in seismic zones.