Mitochondrial quality control (MQC) mechanisms, including proteostasis, mitophagy, mitochondrial dynamics, and biogenesis, are essential for maintaining mitochondrial function and overall cellular health. Dysregulation of these systems is a common feature of both neurodegenerative diseases and cancer, but the outcomes differ. Neurons depend strongly on healthy mitochondria and are easily damaged when MQC fails, resulting in organellar dysfunction and oxidative stress. By contrast, cancer cells often adapt by using MQC pathways to sustain survival and resist cell death. The mitochondrial unfolded protein response (mtUPR) and mitophagy are central to these processes, yet their roles are context-dependent. In neurodegeneration, activation of these pathways may help neurons survive, yet persistent stimulation can shift towards harmful effects. In cancer, these same pathways enhance metabolic flexibility, promote resistance to treatment, and support tumor progression. Although therapeutic strategies targeting MQC are being explored, their translation to the clinic is difficult, partly due to opposite effects in different diseases. The observed inverse epidemiological link between cancer and neurodegeneration may also reflect the distinct regulation of MQC pathways. A clearer understanding of these mechanisms is needed to identify new treatment strategies for disorders that are clinically distinct but share common mitochondrial defects.