Changes in cognitive functioning often result from traumatic brain injury (TBI) and predict other important aspects of psychosocial recovery. Despite this pivotal role, no quantitative review of cognitive functioning across the spectrum of TBI severity has been reported. We therefore conducted a meta-analysis of 39 mostly cross-sectional studies of the cognitive effects of mild head injury (MHI) and moderate-severe TBI from the acute phase through long-term follow-up. The studies reported 48 comparisons of patients (n = 1716) and control subjects (n = 1164). Averaged across all follow-up periods, the effect of moderate-severe TBI (weighted mean Cohen's d = -0.74) was more than three times the effect of MHI (weighted mean d = -0.24) on overall cognitive functioning. Further, the natural logarithm of the follow-up interval correlated very strongly with estimates of d among patients with MHI, but less so among those with moderate-severe TBI. In short, findings from published research suggest that overall cognitive functioning recovers most rapidly during the first few weeks following MHI, and essentially returns to baseline within 1-3 months. Cognitive functioning also improves during the first two years after moderate-severe TBI, but remains markedly impaired even among patients tested > 2 years post-injury.