Longevity is regulated by a network of intimately linked metabolic systems. We used a combination of mouse population genetics and RNAi in C. elegans to identify mitochondrial ribosomal protein S5 ( Mrps5) and other mitochondrial ribosomal proteins (MRPs) as metabolic and longevity regulators. MRP knockdown triggers mitonuclear protein imbalance, reducing mitochondrial respiration and activating the mitochondrial unfolded protein response (UPR mt). Specific antibiotics targeting mitochondrial translation and ethidium bromide, which impairs mitochondrial DNA transcription, pharmacologically mimic mrp knockdown and extend lifespan by inducing mitonuclear protein imbalance, also in mammalian cells. In addition, resveratrol and rapamycin, longevity compounds acting on different molecular targets, similarly induced mitonuclear protein imbalance, UPR mt and lifespan extention in C. elegans. Collectively these data demonstrate that MRPs represent an evolutionary conserved protein family that ties the mitochondrial ribosome and mitonuclear protein imbalance to UPR mt, an overarching longevity pathway across multiple species.