Modal mass is a fundamental factor in many tasks, such as structural vibration analysis, structural health monitoring, seismic assessment and vibration control. For an existing structure, modal testing needs to be conducted to obtain the modal mass. It is not an easy task because accurately measuring the external excitation from a mechanical shaker is not easy. To tackle this problem, the study proposes an approach to identify a structure’s modal mass from its resonant response caused by single person dynamic loads, such as swaying in horizontal direction. The human-induced load is adopted because it is low-frequency, nearly periodic, does not require on-site power-supply, and is easy to apply to a real structure. Ambient vibration testing is first conducted on the target structure to get its frequencies. The structure is then excited by a tester acting at the frequency of the vibration mode of interest. The proposed method identifies the modal mass by particle swarm optimization. Applying the method to a 24-story building shows an identification error of 14.54%. The results demonstrate that the new approach is a convenient, effective, and low-cost method for obtaining modal mass of an existing structure with reasonable accuracy.