Polycrystalline La₂NiMnO₆ thin films are prepared on Pt/Ti/SiO₂/Si substrates by the sol–gel method. Through controlling the processing parameters, the cation ordering can be tuned. The disordered and ordered thin films exhibit distinct differences for crystal structures as well as properties. The crystal structure at room temperature characterized by X-ray diffraction and Raman spectra is suggested to be monoclinic (P2₁/n) and orthorhombic (Pbnm) for the ordered and disordered thin films, respectively. The ferromagnetic–paramagnetic transition is 263 K and 60 K for the ordered and disordered samples respectively, whereas the saturation magnetic moment at 5 K is 4.9 μ_B fu⁻¹ (fu = formula unit) and 0.9 μ_B fu⁻¹. The dielectric constant as well as magnetodielectric effect is higher for the ordered La₂NiMnO₆ thin films. The magnetodielectric effect for the ordered thin film is dominantly contributed to the intrinsic coupling of electric dipole ordering and fluctuations and magnetic ordering and fluctuations, while it is mainly contributed to Maxwell–Wagner (M–W) effects for the disordered thin film. The successful achievements of ordered and disordered polycrystalline La₂NiMnO₆ thin films will provide an effective route to fabricate double-perovskite polycrystalline thin films by the sol–gel method.