A simple and straightforward strategy was developed to fabricate magnetically separable MnFe2O4-graphene photocatalysts with differing graphene content. It was found that graphene sheets were fully exfoliated and decorated with MnFe2O4 nanocrystals having an average diameter of 5.65 nm and a narrow particle size distribution. It is very interesting that, although MnFe2O4 alone is photocatalytically inactive under visible light irradiation, the combination of MnFe2O4 nanoparticles with graphene sheets leads to high photocatalytic activity for the degradation of methylene blue under visible light irradiation. The strong magnetic property of MnFe2O4 nanoparticles can be used for magnetic separation in a suspension system, and therefore it does not require additional magnetic components as is the usual case. Consequently, the MnFe2O4-graphene system becomes a dual function photocatalyst. The significant enhancement in photoactivity under visible light irradiation can be ascribed to the reduction of graphene oxide (GO), because the photogenerated electrons of MnFe2O4 can transfer easily from the conduction band to the reduced GO, effectively preventing a direct recombination of electrons and holes. Hydroxyl radicals play the role of main oxidant in the MnFe2O4-graphene system, and the radicals' oxidation reaction is obviously dominant.