This study evaluated the effects of competitive intensity, represented by the variables time in competition, phase of play, and field location, on injury severity in U.S. high school football. The injury rate was higher in competition than practice (RR = 4.75, 95% CI: 4.34–5.20). Mild and moderate injuries were frequently lower leg/foot/ankle sprains/strains and concussions. Severe injuries were frequently knee Sprains/strains and arm fractures. Severe injuries composed a greater proportion of injuries sustained during the beginning and middle of competition compared with injuries sustained during the end/overtime (IPR = 1.83, 95% CI: 1.25–2.69). Compared with injuries sustained during general play, a greater proportion of kickoff/punt injuries were severe (IPR = 1.69, 95% CI: 1.07–2.68) or were concussions (IPR = 1.86, 95% CI: 1.05–3.30). Identifying factors contributing to severe injury is a crucial first step toward developing targeted evidence-based interventions aimed at reducing the incidence of severe injuries among the millions of high school football players.