Seismic performance assessment of existing buildings requires the existing material's properties to be determined as this has a significant influence on the seismic risk. As one of the most common construction type in Turkey, masonry buildings are assessed generally based on the recommended material properties specified in relevant codes as opposed to testing of samples taken from the existing buildings. However, due to different material types and workmanship quality, the specified properties might be quite different from the existing values. For these reasons, a comprehensive study has been undertaken to investigate the influence of mechanical properties of existing masonry buildings on their seismic performance. The study has two parts. First, the mechanical properties of materials commonly used in the Turkish masonry construction were determined by extracting wallets from existing structures and conducting laboratory experiments. The second part of the study was devoted to seismic performance assessment of selected buildings using material properties determined based on field tests and values specified in the codes. Within the context of determining existing mechanical properties, compression, diagonal tension, and shear tests were conducted on the specimens taken from existing buildings. Stress-strain responses for compression and diagonal tension tests were measured. When compared with the allowable strength values recommended by the current Turkish design code, the test results indicated significant differences from code recommended values. Thus, building assessment results were found to be greatly influenced by the material strength estimations. Based on the evaluation of material test results, lower-bound material properties are proposed for use in assessment of masonry buildings.