Compared to mobile devices of the past decade, current devices provide desktop computer-level processing power in a palm-sized package. However, battery life of such devices has not made the same stride, forcing the user to rely on portable battery packs for backup power. Portable energy harvesting methods could provide a possible solution to reduce the user's dependence on the power grid. In this regard, a mobile energy harvester powered by human arm motion during walking or running activities is developed and tested. The device consists of a pendulum connected to a DC generator through a planetary gear train. An electronic module filters the output and supplies power to a load. A mathematical model of the human arm-harvester system is derived and simulated based on the triple pendulum system. The power output of the harvester for two output modes are recorded for a constant running pace by the user, and the results are compared with the simulation. The model predicted an energy output of 1.72 mJ and testing resulted in 1.39 mJ and 1.16 mJ for a period of 5 s of running activity. This successfully demonstrates the energy harvester's potential as a mobile power supply for charging portable consumer electronics.