Preserving old buildings is essential for maintaining our historical and cultural heritage. These structures showcase unique architectural styles, promote sustainability through material reuse, and improve performance and value by integrating modern and energy-efficient technologies. The mechanical characterization of load-bearing masonry in Portugal presents significant challenges due to the considerable heterogeneity in construction methods and materials used over time. In recent years, numerous flat-jack tests have been conducted across the country, with a special focus on the historic center of Lisbon, covering both civil buildings and historic monuments. Following the masonry typology presented by EC8-3, the tested masonry was classified. The determination of the mechanical properties of masonry, namely the compressive stress value, modulus of elasticity, and shear modulus, was obtained through 79 in situ flat-jack tests. The analysis of the obtained results aims to contribute to the understanding of the mechanical properties of Portuguese masonry, namely the irregular stone masonry, with pebbles, erratic and irregular stone units, present in the historical city center of Lisbon.