Analysis of 7,152 cardiovascular disease-free adults from NHANES III reveals that resting tachycardia (≥100 bpm) increases all-cause mortality risk by 258% and cardiovascular death risk by 105% over 14 years. The study identified subclinical myocardial injury (SCMI) — silent cardiac damage detectable through ECG scoring — in 24% of participants, with tachycardic individuals showing 134% higher odds of harboring this hidden damage. This finding illuminates a previously unclear mechanistic pathway linking elevated heart rate to premature death. The discovery suggests that routine ECG analysis could identify at-risk individuals before symptoms appear, potentially revolutionizing preventive cardiology. However, the observational design cannot establish causation, and questions remain about whether treating elevated heart rate would reduce SCMI or mortality. The mediation effect was modest, indicating other pathways also contribute to tachycardia's mortality risk. As this preprint awaits peer review, the findings require validation before clinical implementation. Still, this represents a significant advance in understanding why a simple vital sign predicts lifespan, potentially enabling earlier intervention in apparently healthy adults.