We report on the physical properties of Mn-rich, nonstoichiometric La₂Ni_0.5Mn_1.5O₆ ferromagnetic insulator, prepared by sol-gel method. The single-phase orthorhombic Pbnm structure for the compound was confirmed by x-ray diffraction measurements. Dc magnetization measurements revealed a high saturation magnetization of ~5.95 μ_B/f.u. at 5 K, and a ferromagnetic to paramagnetic transition at ~162 K. Ac magnetic susceptibility measurements confirmed a broad frequency-dependent anomaly at lower temperatures indicating the presence of spin-glass type magnetic interactions. The ac susceptibility data have been discussed within the framework of the critical slowing down model and Vogel–Fulcher law, and confirmed the cluster spin-glass dynamics with a relaxation time of the order of 10⁻⁵ s. The valence of Ni and Mn ions was verified from the x-ray absorption near edge structure spectroscopy. The origin of cluster spin-glass state was discussed in terms of several possible magnetic exchange interaction pathways among Ni and Mn ions.