Herein, a highly efficient and environmentally benign protocol for the synthesis of biologically important pyrimido[4,5-b]quinolinone-2,4-diones from aromatic amines, barbituric acid and aryl aldehyde is reported. This process takes place at room temperature under direct irradiation from a UV₃₆₅ light source in the absence of a photocatalyst. The reported approach has several advantages such as high yield, clean reaction conditions, chromatography-free synthesis and the use of a cheap water–glycerol solvent system, which is also an environmentally friendly solvent. The applicability of this protocol for the large-scale synthesis of pyrimido[4,5-b]quinolinone-2,4-diones without wasting any expensive chemicals is an added advantage.