Mitochondria are critical for bioenergetics, calcium and redox homeostasis, and apoptosis in neurons. Neurons are especially susceptible to mitochondrial malfunction since they are terminally differentiated and have extraordinary energy requirements. Mechanisms such as defective oxidative phosphorylation, altered fission-fusion dynamics, defective mitophagy, impaired axonal transport and excessive reactive oxygen species (ROS) generation are increasingly appreciated as important mechanisms in major neurodegenerative diseases (NDDs) like Alzheimer’s disease (AD), Parkinson’s disease (PD), amyotrophic lateral sclerosis (ALS) and Huntington’s disease (HD). The review discusses the structure and function of mitochondria with emphasis on oxidative phosphorylation, ROS and reactive nitrogen species (RNS) signalling, calcium-mediated excitotoxicity, lipid metabolism, and apoptotic and non-apoptotic cell-death mechanisms including necroptosis, ferroptosis, pyroptosis and parthanatos. In particular, disease-specific processes are highlighted, such as impaired mitophagy by amyloid β/tau in AD, PTEN-induced kinase 1 (PINK1)/Parkin dysfunction and complex I inhibition in PD, Superoxide Dismutase 1 (SOD1)/C9orf72-associated mitochondrial defects in ALS, and bioenergetic dysfunction by mutant huntingtin in HD. Therapeutic approaches include mitochondria-targeted antioxidants, fusion and biogenesis enhancers, fission inhibitors, inflammasome modulators, Mitochondrial Permeability Transition Pore (mPTP) inhibitors and gene therapy. Also highlighted are recent advancements such as Ubiquitin-Specific Peptidase 30 (USP30) inhibition, intercellular mitochondrial transfer and regulatory approval of a mitochondria-targeted peptide, together with evidence supporting causality and hurdles underpinning clinical translation. Graded against human-genetic, patient-tissue and model-based evidence, mitochondrial dysfunction is best supported as a primary driver in PD and genetic ALS, and as an early amplifier of proteinopathic or mutant-huntingtin toxicity in sporadic AD and HD.