This study develops an easy-to-conduct numerical calculation method to assess the effect of Textile Reinforced Mortar when used as externally bonded flexural strengthening technique in low-grade reinforced concrete beams from ancient structures (aged between 60 and 100 years old). Using materials main mechanical characteristics and the beams dimensions as inputs, the model defines the behaviour of the strengthened elements. This paper includes the experimental characterization of the whole strengthening solution, from the constituent materials and the composite in pure tensile to scaled beams strengthened in flexure. In total three different materials – basalt, carbon and steel cords – are used as Textile Reinforced Mortar inner reinforcement. Tests are carried out on sixteen low-grade reinforced concrete beams that reproduce low-quality concretes with a compression strength below 17 MPa and, in general, poor mechanical properties, as those present in old and decayed structures. This experimental campaign includes the study of two other particular features of the adopted retrofitting technique: the strengthening ratio and the use of an anchorage system. The obtained experimental findings are assessed and satisfactorily compared with the developed numerical approach, leading us to conclude that Textile Reinforced Mortar is an effective solution for retrofitting structures made of low-grade reinforced concretes in terms of increasing their deformation and load-bearing capacity under flexural loads (the maximum bending moment has increased between 30% and 200%). Besides, the failure mechanism due to undesired premature detachment was effectively countered using U-shaped anchors, failing the strengthened beams due to the composite tensile failure.