We report on theoretical and experimental work involving a particular molecular switch, an [FeII(L)2]2+ complex, that utilizes a spin transition (“crossover”). The hallmark of this transition is a change of the spin of the metal ion, SFe=0 to SFe=2, at fixed oxidation state of the Fe ion. Combining density functional theory and first principles calculations, we demonstrate that within a single molecule this transition can be triggered by charging the ligands. In this process the total spin of the molecule, combining metal ion and ligands, crosses over from S=0 to S=1. Three-terminal transport through a single molecule shows indications of this transition induced by electric gating. Such an electric field control of the spin transition allows for a local, fast, and direct manipulation of molecular spins, an important prerequisite for molecular spintronics.