Photocatalysts driven by visible (VIS) and ultraviolet (UV) light are highly efficient in photodegrading organic pollutants. To improve the catalytic activity of cerium (IV) oxide (CeO2), in this study, we introduced silver (Ag) into this compound hydrothermally to form Ag/CeO2 nanocomposites with molar ratios 1:1, 1:2, and 1:3. We also used the hydrothermal method to add various graphene contents (3, 5, and 10 wt%) to the optimum Ag/CeO2 catalyst. We employed the synthesized nanocomposites as catalysts to remove methylene blue (MB) under irradiation with light and ultrasound. Photocatalytic studies revealed that among the Ag/CeO2 and Ag/CeO2/graphene composites, a molar ratio of 1:2 Ag to CeO2 and a 5 wt% graphene concentration are optimum; i.e., they demonstrate the best degradation efficiency. The degradation efficiency increases further when we use ultrasound irradiation to assist in the photocatalytic process. Moreover, the Ag/CeO2/graphene composites are stable and reusable. We have confirmed that holes play the main role during the photocatalytic process, and we have identified the major intermediate products. We have also proposed a possible degradation pathway for MB.