We propose the use of an electric field stimulus to strongly affect the optical properties of ferromagnetic mixed-valence (MV) dimers. This proposal is based on the prediction of an anomalous Stark effect in the intervalence absorption bands of these multi-electron MV systems. As distinguished from the conventional Stark effect observed in one-electron dimers, a strong change of the intervalence bands accompanies the crossing of the different spin levels caused by the application of an electric field. This new effect can be referred to as giant spin-dependent Stark effect. In spintronics this opens up the possibility for optical detection of the spin state in these magnetic molecules.