We fabricated a non-local spin valve device with Co-MgO injector/detector tunnel contacts on a graphene spin channel. In this device, the spin polarization of the injector contact can be tuned by both the injector current bias and the gate voltage. The spin polarization can be turned off and even inverted. This behavior enables a spin transistor where the signal is switched off by turning off the spin injection using the field-effect. We propose a model based on a gate-dependent shift of the minimum in the graphene density of states with respect to the tunneling density of states of cobalt, which can explain the observed bias and gate dependence. Published by AIP Publishing.