Highly efficient one‐pot synthesis of a series of substituted 3‐methylene‐isoindolinones (3 a–x), bis(isoindolinone)ethanes (4 a–d) and 3‐hydroxy‐isoindolinones (6 a–h) derivatives is portrayed from easily accessible starting materials such as 2‐cyanobenzaldehydes (1 a–h) and different aryl/heteroaryl methyl ketones (2 a–k). The structure of Z‐isomer of 3‐methylene‐isoindolinone 3 x was confirmed by single crystal X‐ray analysis. The reaction mechanism studies by control experiments and using O18‐labelled water revealed that the reaction was initiated by attack of a conjugate base of aryl/heteroaryl methyl ketones (2 a–k) to aldehyde group of 1 a–h followed by intramolecular oxidative cyclization involving nitrile functionality to furnish isoindolinone derivatives in good to excellent yields.