Objective: Post concussive syndromes (PCS) is common after mild-TBI, yet are not well studied on a population level. This study examined PCS symptoms, including reliable change over time in a population-based sample up to one year post-TBI. Methods: Prospective follow-up of 527 adults (≥16 years) with mild TBI (mTBI) and assessment data (Rivermead Post concussion Questionnaire; RPQ) at baseline, 1, 6, and/or 12-months post-TBI. Change in mean scores and clinically significant change across RPQ items for each person was calculated between assessment time points using a reliable change index (RCI). Results: While prevalence of all symptoms reduced over time, >30% of participants reported fatigue, slowed thinking, and forgetfulness 12-months postinjury. Using the RCI, <12% of individuals improved from baseline to 1-month, 50% from 1 to 6-months, and 4.2% from 6 to 12-months. Conclusions: Improvements in PCS post-mTBI were most obvious between 1 and 6-months, suggesting lengthy recovery trajectory. A third of patients experience residual cognitive problems 12-months following a mTBI, and while many individuals improve post-TBI, a large proportion remain stable or worsen.