Coenzyme Q10 (CoQ10) is biologically plausible as an ergogenic aid through roles in mitochondrial energy production and antioxidant defence, yet findings from randomised trials are inconsistent. This review included twenty-four studies from six databases published up to November 2025, assessing effects of CoQ10 on exercise performance, subjective fatigue and circulating CoQ10 levels in healthy adults. Randomised trials comparing CoQ10 with placebo were synthesised using a three-level model. Risk of bias was assessed with RoB2 and certainty judged with Grading of Recommendations Assessment, Development, and Evaluation. Supplementation consistently increased blood CoQ10, indicating robust biochemical responsiveness. In contrast, performance effects were small and inconsistent. In primary analyses, chronic supplementation showed a small benefit, whereas acute supplementation showed no benefit. After excluding outliers, the chronic effect was no longer stable and the acute effect remained trivial. All subgroup analyses were restricted to chronic supplementation. Within this context, aerobic endurance was significant in primary analyses but became borderline after outlier exclusion, while anaerobic and strength outcomes showed little change. Evidence for reduced subjective fatigue was suggestive and became more consistent after outlier exclusion. Benefits in trained individuals were unstable and became consistent only after outlier exclusion. No stable dose–response pattern emerged for supplementation dosage or duration. Heterogeneity and moderate-to-high risk of bias reduced certainty. Collectively, CoQ10 reliably elevates circulating levels but provides at most modest and context-dependent benefits for exercise performance, largely under chronic use. Overall, certainty is very low to low. Well-controlled randomised trials that standardise formulation, dose and duration and examine sex-specific and endurance-related responses are needed.