For the first time, nanocrystalline photocatalysts of spinel MgFe2O4, ZnFe2O4 and orthorhombic CaFe2O4 oxides were synthesized (at low temperature 973 K) by microwave sintering, in one sixtieth of the time required to that of the conventional method. A significantly improved crystallinity was obtained for the samples irradiated for longer duration of time (10–100 min). The theoretically computed electronic structure of the MFe2O4 (M: Ca, Zn, Mg) systems was respectively correlated with the experimental results obtained from their structural and photocatalytic characterization. The photocatalytic performance was found to be affected by surface area and crystallinity of the photocatalyst. The density functional theory (DFT) calculations of MFe2O4 lattices revealed that M-ion controllably affects the density of sates of the Fe–d orbitals near the Fermi level. Consequently they play an important role in determining the band-energetics and thus the visible light photocatalytic activity for methylene blue degradation.