Aryl diazonium salts are versatile compounds known for their reactivity in various transformations. In this study, we explored the arylation of 3-methylene isoindolinones using aryl diazonium salts. The objective was to develop a method for accessing 3-benzyl-3-methoxyisoindolin-1-ones, a class of compounds with diverse biological activities and synthetic importance. Optimization of the reaction conditions revealed the significance of light irradiation for improved yields. The substrate scope investigation demonstrated the compatibility of various diazonium salts and N-substituents on the 3-methylene isoindolinone scaffold, yielding the corresponding methoxy lactams with high yields. Mechanistic insights were obtained by observing the effect of base and light irradiation. Two initiation mechanisms were proposed: in the presence of a base, the reaction could proceed even without light irradiation, while light irradiation was necessary in the absence of a base. DFT calculations were performed to elucidate the mechanism of the reaction and provide energetic considerations