Alzheimer's disease (AD) and inclusion body myopathy (IBM) are common and devastating diseases characterized by the aggregation of amyloid-β peptide (Aβ), yet we know relatively little about their underlying molecular mechanisms or how to treat them. Here, we provide bioinformatic and experimental evidence of a conserved mitochondrial stress response signature present in Aβ proteotoxic diseases in human, mouse and C. elegans, and which involves the UPRmt and mitophagy pathways. Using the worm model of Aβ proteotoxicity, GMC101, we recapitulated mitochondrial features and confirmed the induction of this mitochondrial stress response as key to maintain mitochondrial proteostasis and health. Importantly, boosting mitochondrial proteostasis by pharmacologically and genetically targeting mitochondrial translation and mitophagy increases fitness and lifespan of GMC101 worms and reduces amyloid aggregation in cells, worms, and in aged and AD transgenic mice. Our data support the relevance of enhancing mitochondrial proteostasis to delay Aβ proteotoxic diseases, such as AD and IBM.