Manganese added nickel oxide composite nanoparticles (1 − x)NiO-xMnO2 (x = 0.00, 0.5, 0.10, 0.15 & 1.0) were prepared using mechanical milling and subjected to different characterization techniques to study their structural, optical and magnetic properties. The microstructures, surface morphology, crystallite size of the nanoparticles were studied using X-ray diffractometer (XRD), field emission scanning electron microscopy (FE-SEM). From these it was found that the nanoparticles were in cubic structure. Secondary phases were observed with increase of Mn concentration. The crystallite size of the particles was calculated using Scherer's relation and found that the nanoparticles were in the range of 25–35 nm. The magnetic properties of the composite nanoparticles were studied at room temperature and at 100 K using vibrating sample magnetometer (VSM) in the applied field range of ±75 kOe. The MnO2 nanoparticles exhibited paramagnetic nature at room temperature. The NiO nanoparticles exhibited soft ferromagnetic nature and strength of magnetization increased with applied external magnetic field (±50 kOe). The Mn doped nickel oxide composite nanoparticles also exhibited soft ferromagnetism at room temperature and at 100 K. The strength of magnetization increased with increase of Mn concentration and the composite nanoparticles at x = 0.15 (Ni0.85Mn0.15O) shown the highest magnetic moment of 6 emu/g at 100 K. The temperature dependent magnetic moments for (1 − x)NiO-xMnO nanoparticles at different x values were also discussed in detail. © 2018 Elsevier B.V.