<p>Understanding material properties is fundamental to the structural assessment of existing buildings. In the case of masonry constructions, this characterisation is particularly challenging due to the high variability in materials, construction techniques, and the influence of ageing and previous interventions. These challenges are exacerbated by the limitations of in-situ testing, particularly in heritage structures, where intrusive methods are often restricted. Over recent decades, the Portuguese technical and academic communities have made substantial efforts to characterise the mechanical behaviour of masonry structures. However, these initiatives have been mainly fragmented and uncoordinated, limiting their potential to inform national and international standards. This paper presents part of the ongoing work of the Working Group (WG) under Sub-commission 8 (Eurocode 8) of CT115 (Portuguese Technical Commission for Structural Eurocodes) aimed at creating the basis for the proposal of mechanical properties of masonry typologies to be included in the future generation of the Portuguese Annexe of Part 3 of Eurocode 8. With that objective in view, the group has compiled and systematised data on the mechanical properties of masonry elements from academic research and professional practice into a unified, freely accessible database. A key outcome of this first stage of the work is the proposal of new analytical expressions that correlate mechanical parameters—compressive strength, shear strength, elastic modulus, and shear modulus—with the Masonry Quality Index (MQI), a semi-quantitative indicator of masonry construction quality. Such a task, which constitutes the core of this paper, will contribute directly to the main objective of the WG by providing a practical basis for the structural assessment and, with that, supporting a more robust and harmonised evaluation of Portugal’s masonry building stock.</p>