A transient study of the cooling process inside a cabinet is presented in this work. Computational fluid dynamics (CFD) is used to obtain the transient temperature of the air inside a small cabinet. Three cooling levels, named Case 1, Case 2, and Case 3, were studied under different lapses of time and operational conditions. The temperature of the evaporator plate and the temperature of the room change with time through the implementation of user-defined functions (UDFs). The buoyancy effects that occur inside the cabinet (natural convection) were modeled using the approximation of all the properties fitted to temperature polynomials (PFTP). The transient temperature of the air inside the cabinet was obtained, until the second ON/OFF stage is reached, for the three cases studied. The prediction of the transient temperature of the air inside the small cabinet was validated with experimental data. The average relative errors of the transient temperatures of the air inside the cabinet were 0.49%, 0.19%, and 0.13% for Case 1, Case 2, and Case 3, respectively. The behavior of the temperature and velocity distributions of the air inside the cabinet for the ON and OFF stages is obtained. Finally, a better range of the temperature for the preservation of food, medicines, and biological matter is obtained with an increase in the thickness of the insulation material. An increment of 14.9% of the removed energy was obtained inside the cabinet with 12 cm of the insulation material, and this increment is related to the case with no insulation material. The results of this work could help in improving the design and performance of the cabinets in further works.