Mitochondrial fission is a highly regulated process that, when disrupted, can alter metabolism, proliferation and apoptosis ^1-3 . Dysregulation has been linked to neurodegeneration ^3,4 , cardiovascular disease ³ and cancer ⁵ . Key components of the fission machinery include the endoplasmic reticulum ⁶ and actin ⁷ , which initiate constriction before dynamin-related protein 1 (DRP1) ⁸ binds to the outer mitochondrial membrane via adaptor proteins ^9-11 , to drive scission ¹² . In the mitochondrial life cycle, fission enables both biogenesis of new mitochondria and clearance of dysfunctional mitochondria through mitophagy ^1,13 . Current models of fission regulation cannot explain how those dual fates are decided. However, uncovering fate determinants is challenging, as fission is unpredictable, and mitochondrial morphology is heterogeneous, with ultrastructural features that are below the diffraction limit. Here, we used live-cell structured illumination microscopy to capture mitochondrial dynamics. By analysing hundreds of fissions in African green monkey Cos-7 cells and mouse cardiomyocytes, we discovered two functionally and mechanistically distinct types of fission. Division at the periphery enables damaged material to be shed into smaller mitochondria destined for mitophagy, whereas division at the midzone leads to the proliferation of mitochondria. Both types are mediated by DRP1, but endoplasmic reticulum- and actin-mediated pre-constriction and the adaptor MFF govern only midzone fission. Peripheral fission is preceded by lysosomal contact and is regulated by the mitochondrial outer membrane protein FIS1. These distinct molecular mechanisms explain how cells independently regulate fission, leading to distinct mitochondrial fates.