The present work documents construction of C4-cyanoalkylated phthalazinones under visible light-mediated conditions. The three-component reaction involving aryl hydrazines, 2-formylbenzoic acids and cyclobutanone oxime esters with organic dye Eosin Y as photocatalyst, features initial formation and subsequent cyanoalkylation of phthalazin-1(2H)-one in one pot. The reaction is notable for mild conditions, operational simplicity, wide substrate scope and good yields of the products. The phthalazinone products were further converted into other valuable organic derivatives.