The aim to retrofit and preserve the monumental stone masonry buildings due to their historical and cultural relevance is accompanied by the necessity of understanding the behaviour of the unstrengthen structure, as well as its behaviour after the strengthening systems are applied. There is scarce information related to the mechanical properties of stone masonry buildings and even less regarding the assessment of these characteristics in numerical models. Therefore, simulating the force displacement variation and the stress-strain distribution of stone masonry loaded in diagonal compression is a challenging issue. This work contributes to this topic by developing two detailed micro non-linear 3D models. The first model was designed for an unreinforced masonry (URM) wall and the second one was developed for a strengthened URM wall. For this purpose, a commonly used seismic strengthening system, referred to as reinforced plastering mortar (RPM) or textile reinforced mortar (TRM) was applied on the wall. All the components of the TRM strengthening system and the interfaces between the system and the stone masonry wall were considered in the numerical model. The structural responses of the models were analysed and compared and the TRM system effectiveness in increasing the in-plane load resistance and ductility of stone masonry walls was highlighted.