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The mechanisms of mitochondrial homeostasis in neurons—‘mitostasis’—are enigmatic and disease-prone, given a neuron’s large synaptic pool of mitochondria. To better understand mitostasis in mature mouse motor axons, we used ex vivo optical pulse-chase measurements of mitochondrial volume flux and identified a reiterative degradation system near presynaptic terminals and distal paranodes, which accounts for 75% of mitochondrial turnover in synapses. This distal filter system captures dysfunctional mitochondria from the retrograde stream and redirects them for lysosomal degradation. It depends on optineurin, a motor neuron disease-related mitophagy adaptor, but not on PINK1 and parkin, implicating a non-canonical mitophagy pathway. In presymptomatic motor neuron disease models, where retrograde transport is disrupted, the fraction of removed mitochondria is increased. Thus, we identify a new mitostasis system in distal axons with a cascade of checkpoints for local mitophagy, which normally maintains mitochondrial mass balance but is disrupted early in degenerative axonopathies.