Despite longstanding scientific interest in the centrality of mitochondria to biological aging, directly controlling mitochondrial function to test causality has eluded researchers. We show that specifically boosting mitochondrial membrane potential through a light-activated proton pump reversed age-associated phenotypes and extended C. elegans lifespan. We show that harnessing the energy of light to experimentally increase mitochondrial membrane potential during adulthood alone is sufficient to slow the rate of aging.