A model is proposed that describes plastic deformation in metal/graphene composites with a bimodal grain size distribution of the metallic matrix. Within the model, dislocation pile-ups are generated in large grains at Frank-Read sources, and their stresses promote dislocation motion within the nanocrystalline/ultrafine-grained phase. Also, the presence of graphene gives rise to the mechanisms of strengthening, such as the load transfer to graphene platelets, thermal-mismatch-induced strengthening and Orowan strengthening, as well as to back stress hardening. We demonstrated that the strengthening and strain hardening in bimodal metal/graphene composites are dominated by the Orowan strengthening and back stress hardening. The results also indicate that regardless of the lateral size of graphene platelets, bimodal metal/graphene composites can simultaneously have high yield strength and large uniform deformation but the values of the yield strength and critical uniform deformation are higher in the case of small graphene platelets.