The metal hydrazine precursor of Ni0.9Mn0.1Fe2O4 nanoparticles was prepared through co-precipitation technique. Infrared spectrum of the precursor was showed the bridging bidentate nature of hydrazine. Multistep endothermic mass loss was observed from the thermogravimetric–differential thermogravimetric curves of the precursor. Ni0.9Mn0.1Fe2O4 nanoparticles synthesized from the precursor using thermal decomposition technique were characterized by infrared spectroscopy, energy-dispersive spectroscopy, powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, vibrating sample magnetometry and UV-diffuse reflectance spectroscopy which proposed the phase structure, morphology, magnetic and optical properties. An effective photocatalytic activity has shown by Ni0.9Mn0.1Fe2O4 nanoparticles on Congo red, malachite green, methylene blue, methyl red, rhodamine B and rose bengal under direct sunlight at different time intervals. The nanoparticles were magnetically separated and showed good recycling ability.