Mitochondria are key to cellular energetics, metabolism, and signaling. Their dysfunction is linked to devastating diseases including mitochondrial disorders, diabetes, neurodegenerative diseases, cardiac disorders, and cancer. Here, we present a novel knockout mouse model lacking the complex IV assembly factor SMIM20/MITRAC7. SMIM20-/- mice display cardiac pathology with reduced heart weight and cardiac output. Heart mitochondria present with reduced levels of complex IV associated with increased complex I activity, have altered fatty acid oxidation, and display elevated levels of ROS production. Interestingly, mutant mouse cardiomyocytes show unphysiological Ca2+ handling, which can be attributed to the increase in mitochondrial ROS production. Our study presents a new example of a tissue-specific phenotype in the context of OXPHOS dysfunction. Moreover, we identify a molecular link between complex IV dysfunction and Ca2+ handling at the endoplasmic reticulum through ROS signalling.