Substitution with rare earth (RE3+) and transition (X = Cu, Ni, Mn, Zn, etc.) metals can distort the spinel ferrite structure and create defects, which can be exploited in multifarious applications. In this study, yttrium and nickel co-substituted cobalt ferrites [Co1-xNixFe1.85Y0.15O4], (0.0 ≤ x ≤ 0.15) were synthesized by a modified sol-gel reaction route. The X-ray diffraction (XRD) patterns confirmed the single phase and highly crystalline nature of the prepared samples. Field emission scanning electron microscopy with energy dispersive X-ray (FESEM-EDX) spectroscopy confirmed the morphology of the prepared samples sintered at 800 °C and showed the grain sizes of the particles decreased with the addition of nickel ions. The simultaneous co-existence of saturation magnetization and ferroelectricity at room temperature confirmed the multiferroic nature of the co-substituted cobalt ferrites. The dielectric and impedance studies confirmed the Maxwell-Wagner polarization phenomenon and the enhanced values with Y-Ni co-substitution. The maximum saturation ferroelectric polarization was attained around 6.142 μC cm-2 for yttrium-substituted cobalt ferrite and then decreased with the increase in the substitution % of nickel. The maximum value of saturation magnetization (Ms = 99.50 emu g-1) was obtained with the highest substitution % of nickel of x = 0.15 in yttrium-substituted cobalt ferrite (YCFO) nanoferrites, which was also confirmed from the vibrating sample magnitude (VSM) studies at room temperature. The improvements in the structural, morphological, electrical, and multiferroic properties in the co-substituted cobalt ferrites were found to correlated to the substitution of the bigger cationic Y3+ ions compared to the Fe3+ ions, while the small substitution of Ni2+ ions lead to changes in the lattice parameters, porosity, and conduction behavior. These significant enhancements in co-substituted cobalt ferrites can be exploited for hydroelectric cell applications.