BACKGROUND: Reduced balance, spasticity, contractures, muscle weakness, and motor skill levels may all contribute to mobility limitations after traumatic brain injury (TBI), yet the key physical impairments that contribute to mobility limitations remain unclear. OBJECTIVE: The aim of this study was to determine which physical impairments best predict mobility performance after a period of 6 months of rehabilitation. PARTICIPANTS: Participants with TBI were selected if they were receiving therapy for mobility limitations but were able to walk without physical assistance. OUTCOME MEASURES: The clinical assessment included measures of balance, spasticity, and contracture, and 3-dimensional quantitative gait analysis was used to quantify joint power generation and motor skill level on 31 adults with severe TBI. Mobility outcome was quantified with the high-level mobility assessment tool. RESULTS: Two variables, ankle joint power generation during the push-off phase of gait and motor skill level, explained 66.5% of the variability in mobility outcome. Balance, strength, and mobility performance, all improved significantly over the 6 months of rehabilitation. Only 2 participants had contractures, which affected mobility. Balance disorders were prevalent and improved with rehabilitation, yet they contributed to only a limited extent to the level of recovery in mobility. CONCLUSION: Ankle joint power generation at push-off was the strongest predictor of mobility outcome after 6 months of rehabilitation in ambulant people with TBI.