The objective was to clarify occurrence, phenomenology, and risk factors for novel psychiatric disorder (NPD) in the first 3 months after mild traumatic brain injury (mTBI) and orthopedic injury (OI). Children aged 8-15 years with mTBI (n = 220) and with OI but no TBI (n = 110) from consecutive admissions to an emergency department were followed prospectively at baseline and 3 months post-injury with semi-structured psychiatric interviews to document the number of NPDs that developed in each participant. Pre-injury child variables (adaptive, cognitive, and academic function, and psychiatric disorder), pre-injury family variables (socioeconomic status, family psychiatric history, and family function), and injury severity were assessed and analyzed as potential confounders and predictors of NPD. NPD occurred at a significantly higher frequency in children with mTBI versus OI in analyses unadjusted (mean ratio [MR] 3.647, 95% confidence interval [CI₉₅] (1.264, 15.405), p = 0.014) and adjusted (MR = 3.724, CI₉₅ (1.264, 15.945), p = 0.015) for potential confounders. In multi-predictor analyses, the factors besides mTBI that were significantly associated with higher NPD frequency after adjustment for each other were pre-injury lifetime psychiatric disorder [MR = 2.284, CI₉₅ (1.026, 5.305), p = 0.043]; high versus low family psychiatric history [MR = 2.748, CI₉₅ (1.201, 6.839), p = 0.016], and worse socio-economic status [MR = 0.618 per additional unit, CI₉₅ (0.383, 0.973), p = 0.037]. These findings demonstrate that mild injury to the brain compared with an OI had a significantly greater deleterious effect on psychiatric outcome in the first 3 months post-injury. This effect was present even after accounting for specific child and family variables, which were themselves independently related to the adverse psychiatric outcome.