The structural, electronic, and magnetic properties of a sol-gel prepared series of La₂Ni_1-xS_cxMnO₆ compounds have been extensively studied using x-ray diffraction, x-ray absorption near edge structure, and dc magnetization techniques, respectively. The entire series was isostructural and exhibited the La₂NiMnO₆ double perovskite P2₁/n monoclinic structure. The nonmagnetic Sc³⁺ substitution led to the evolution of competing magnetic phases in La₂Ni_1-xScxMnO₆. The substitution also caused an increase in lattice parameters, cell volume, and bond lengths. Consequently, Sc³⁺ dilution resulted in a dramatic decrease in Curie temperature, suggesting a reduction in the strength of the Ni²⁺ -O^2- -Mn⁴⁺ superexchange ferromagnetic interaction. The Sc³⁺ substitution generated antisite defects, which significantly suppressed the saturation magnetization of the system. The competing magnetic interactions observed in the La₂Ni_1-xScxMnO₆ system are discussed in terms of cation disorder, cation valances, and changes in the bond lengths / angles, caused by the Sc³⁺ substitution.