We characterized the prevalence, mechanisms, and sex difference of lifetime traumatic brain injury (TBI) in a precariously housed sample. We also examined the impact of TBI severity and timing on becoming and staying homeless. 285 precariously housed participants (adults n = 226, youths n = 59) completed the Brain Injury Screening Questionnaire (BISQ) in addition to other health assessments. A history of TBI was reported in 82.1% of the sample, with 64.6% reporting > 1 TBI, and 21.4% reporting a moderate or severe TBI (msTBI). 10.1% of adults had traumatically-induced lesions on MRI scans. Assault was the most common mechanism of injury overall, and females reported significantly more TBIs due to physical abuse than males (adjusted OR = 1.26, 95% CI = 1.14 - 1.39, p = 9.18e-6). The first msTBI was significantly closer to the first experience of homelessness (b = 2.79, p = 0.003) and precarious housing (b = 2.69, p = 7.47e-4) than was the first mild TBI. Traumatic brain injuries more proximal to the initial loss of stable housing were associated with a longer lifetime duration of homelessness (RR = 1.04, 95% CI = 1.02 - 1.06, p = 6.8e-6) and precarious housing (RR = 1.03, 95% CI = 1.01 - 1.04, p = 5.5e-10). These findings demonstrate the high prevalence of TBI in vulnerable persons and the severity- and timing-related risk that TBI may confer for the onset and prolongation of homelessness.