As a variety of wearable electronics with different functions have emerged, multifunctional devices with self-powered nature are desired for solving both problems of the tighter on-body wearing space and energy budget. Herein, we propose an eccentric-rotor-based multifunctional wearable electromagnetic vibration energy harvester (EMVEH) to function as an ultralow-frequency biomechanical energy scavenging device and a self-powered motion sensor for human activity recognition (HAR) at different positions, respectively. Kinetic simulations based on Euler-Lagrange equations provide a detailed understanding of its vibration pickup performance for effective energy scavenging and human-motion sensing. Results of comprehensive bench tests further verify its adaptability to excitations with ultralow frequencies (<5 Hz) and different directions, and the capability of mechanical excitation perception. Kinetic energy in limb motions during walking and running has been scavenged by a fabricated prototype into electric energy up to $221.49~\mu$ W on average, whereas variations in multiple-feature-based profiles of output voltages in response to these human activities reflect repeatable and human-activity-related voltage patterns. On this basis, walking at 2–4 km/h and running at 4–8 km/h on a treadmill are recognized by these features and trained random forest (RF) classification models when the prototype is worn at the wrist, elbow, and ankle, respectively, all achieving high accuracies over 90%. Mutual constraints between dual design purposes of this multifunctional EMVEH are also fully discussed and released. Thus, the wearing-position-independent HAR performance combined with the efficient energy scavenging capability of our EMVEH expands the application range of wearable EMVEHs to more flexible and practical uses.