The study of magneto-optical (MO) properties is reported in epitaxial cerium (Ce) substituted yttrium iron garnet thin films deposited on Gd₃Ga₅O₁₂ (GGG) substrates using pulsed laser deposition technique at (i) different O₂ partial pressure (OPP) during film deposition and (ii) with different film thickness at a given OPP. The MO activity is estimated by measuring the Kerr rotation using the longitudinal magneto-optical Kerr effect with blue (405 nm), green (532 nm), and red (632 nm) incident laser light. The Kerr rotation is observed to increase with an increase in OPP and vary differently with thickness for a given OPP as a function of incident light wavelength. The results are explained in terms of the possible charge transfer mechanism for the films prepared with OPP variation and the optical interference effects in dielectric films with thickness variation.