Abstract: OBJECTIVE: To examine time from injury to initiation of surgical care and association with survival in US military casualties. BACKGROUND: Although the advantage of trauma care within the "golden hour" after an injury is generally accepted, evidence is scarce. METHODS: This retrospective, population-based cohort study included US military casualties injured in Afghanistan and Iraq, January 2007 to December 2015, alive at initial request for evacuation with maximum abbreviated injury scale scores ≥2 and documented 30-day survival status after injury. Interventions: (1) handoff alive to the surgical team, and (2) initiation of first surgery were analyzed as time-dependent covariates (elapsed time from injury) using sequential Cox proportional hazards regression to assess how intervention timing might affect mortality. Covariates included age, injury year, and injury severity. RESULTS: Among 5269 patients (median age, 24 years; 97% males; and 68% battle-injured), 728 died within 30 days of injury, 68% within 1 hour, and 90% within 4 hours. Only handoffs within 1 hour of injury and the resultant timely initiation of emergency surgery (adjusted also for prior advanced resuscitative interventions) were significantly associated with reduced 24-hour mortality compared with more delayed surgical care (adjusted hazard ratios: 0.34; 95% CI: 0.14-0.82; P = 0.02; and 0.40; 95% CI: 0.20-0.81; P = 0.01, respectively). In-hospital waits for surgery (mean: 1.1 hours; 95% CI; 1.0-1.2) scarcely contributed ( P = 0.67). CONCLUSIONS: Rapid handoff to the surgical team within 1 hour of injury may reduce mortality by 66% in US military casualties. In the subgroup of casualties with indications for emergency surgery, rapid handoff with timely surgical intervention may reduce mortality by 60%. To inform future research and trauma system planning, findings are pivotal.