We present a resonant x-ray diffraction study of the magnetic order in NdFe3(BO3)4 and its coupling to applied electric fields. Our high-resolution measurements reveal two different coexisting magnetic phases, which can directly be controlled and manipulated by external electric fields. More specifically, the volume fraction of the collinear magnetic phase is found to strongly increase at the expense of helically ordered regions when an electric field is applied. These results confirm that the collinear magnetic phase is responsible for the ferroelectric polarization of NdFe3(BO3)4 and, more importantly, demonstrate that coexisting magnetic phases provide a route towards materials with a strong but yet hysteresis-free magnetoelectric response.