Ambient mechanical vibrations are an untapped yet attractive energy source for powering wireless sensor nodes in the upcoming Internet-of-Things. Here we demonstrate the magnetically induced frequency tuning effect in a MEMS electromagnetic vibrational energy harvester. Spiral-shaped springs and double-layer copper micro-coils are fabricated on silicon substrate using MEMS fabrication processes. Numerical simulations and finite-element analysis exhibit substantial transformation in the potential energy and stiffness profiles due to controlled changes in the magnetic repulsion force between the transducing and tuning magnets, which effectively modifies the frequency response profile. Specifically, by increasing the repulsive interaction between the transducing and tuning magnets, both the linear and nonlinear frequency response profiles can be shifted toward higher frequencies. This experimentally validated magnetic tuning mechanism can potentially be implemented in MEMS vibrational energy harvesters with other transduction mechanisms and in other micro-mechanical oscillators for broader frequency response tunability.