General intelligence or g is one of the most thoroughly studied concepts in the behavioral sciences. Measures of intelligence are predictive of a wide range of educational, occupational, and life outcomes, including creative productivity and are systematically related to physical health and successful aging. The nexus of relations suggests 1 or several fundamental biological mechanisms underlie g, health, and aging, among other outcomes. Cell-damaging oxidative stress has been proposed as 1 of many potential mechanisms, but the proposal is underdeveloped and does not capture other important mitochondrial functions. I flesh out this proposal and argue that the overall efficiency of mitochondrial functioning is a core component of g; the most fundamental biological mechanism common to all brain and cognitive processes and that contributes to the relations among intelligence, health, and aging. The proposal integrates research on intelligence with models of the centrality of mitochondria to brain development and functioning, neurological diseases, and health more generally. Moreover, the combination of the maternal inheritance of mitochondrial DNA (mtDNA), the evolution of compensatory nuclear DNA, and the inability of evolutionary processes to purge deleterious mtDNA in males may contribute to the sex difference in variability in intelligence and in other cognitive domains. The proposal unifies many now disparate literatures and generates testable predictions for future studies. (PsycINFO Database Record (c) 2018 APA, all rights reserved).