Cobalt substituted manganese nanocrystalline spinel ferrites having general formula CoxMn1-xFe2O4 was successfully synthesized using hydrothermal method. The structure, magnetic and dielectric properties of as-synthesized samples was characterized through different techniques such as XRD, FESEM, EDX, FTIR, PPMS and Dielectric spectroscopy. X-ray diffraction (XRD) studies showed that the samples have pure cubic spinel phase. The lattice parameter enhances with Co substitution. The SEM images of CoxMn1-xFe2O4 ferrite show that the grain size decreases with an increase in the Co content and the average nanocrystalline sizes were found to be less than 100 nm. Compositional stoichiometry was confirmed by energy dispersive analysis of the X-ray (EDAX) technique. The FTIR spectra reveled two prominent frequency bands in the wave number range 400–600 cm–1 which confirm the cubic spinel structure and completion of chemical reaction. As the cobalt concentration increases, the magnetization of the octahedral sites and hence the net magnetization decreases. It is also observed that the saturation magnetization (Ms), decrease while coercivity (Hc) increase with increase in cobalt substitution. Frequency dependence of dielectric constant shows dielectric dispersion due to the Maxwell–Wagner type of interfacial polarization. AC conductivity measurements suggest that the conduction is due to small polaron hopping.