Nickel-cobalt (Ni-Co) alloy coatings were deposited on mild steel surface from the alkaline bath with triethanolamine as the complexing agent. The effect of cobalt content was investigated on phase structure, surface morphology, mechanical properties and corrosion behaviour of the composite coating. It was observed that the cobalt content in the coatings increased correspondingly with increasing cobalt concentration in the bath. X-ray diffraction (XRD) patterns revealed that peak intensities were significantly varied by adding cobalt in the electrolyte. The phase structure of the deposits was changed from face-centred cubic (FCC) to FCC and hexagonal closed packed (HCP). The crystallite size became smaller at high Co content coatings, which produced smaller grains in their surface morphologies. It was found that higher Co content generated coatings with better microhardness values due to the grain refinement effect by modification of grain size. The adhesion strength of the coatings also increased with high Co content, with the critical load for cracking and chipping increasing with increasing Co content. However, high Co content tends to diminish the corrosion resistance of the coating due to phase structure and grain boundary. © 2019, Science and Technology Research Institute for Defence.