Mitochondria, once viewed mainly as cellular powerhouses, are now recognised as key regulators of cancer metabolism, redox balance, and immune interactions. While early models emphasised a switch to aerobic glycolysis, many tumours exhibit metabolic plasticity and retain oxidative phosphorylation capacity. Mitochondrial DNA (mtDNA) mutations are common across cancers, yet their roles in carcinogenesis and therapy response remain unclear. Emerging base-editing technologies now enable modelling of these mutations, allowing the exploration of their impact on tumourigenesis, which may differ depending on mutation type, heteroplasmy, and tissue origin. mtDNA alterations also shape immune responses within the tumour microenvironment and therefore may influence treatment sensitivity. This review integrates recent advances on mtDNA's role in cancer biology and explores therapeutic opportunities for targeting mitochondrial metabolism.