Austinol, a fungal meroterpenoid derived from 3,5-dimethylorsellinic acid, has a unique chemical structure with a remarkable spiro-lactone ring system. Despite the recent identification of its biosynthetic gene cluster and targeted gene-deletion experiments, the process for the conversion of protoaustinoid A (2), the first tetracyclic biosynthetic intermediate, to the spiro-lactone preaustinoid A3 (7) has remained enigmatic. Here we report the mechanistic details of the enzyme-catalyzed, stereospecific spiro-lactone ring-forming reaction, which is catalyzed by a non-heme iron-dependent dioxygenase, AusE, along with two flavin monooxygenases, the 5'-hydroxylase AusB and the Baeyer-Villiger monooxygenase AusC. Remarkably, AusE is a multifunctional dioxygenase that is responsible for the iterative oxidation steps, including the oxidative spiro-ring-forming reaction, to produce the austinol scaffold.