A Mn(III)-catalyzed cascade cyclization of o-acyl aromatic isocyanides with boronic acids was carried out via a single-step route to generate a series of 3-hydroxyindolenines under mild conditions. A plausible mechanism involving a cascade transmetalation, nucleophilic addition and intramolecular cyclization sequence was proposed and confirmed. In addition, the reaction procedure featured simple synthesis, wide substrate scope, and excellent functional group compatibility.