Although most patients with a mild traumatic brain injury (mTBI) recover within days to weeks, some experience persistent physical, cognitive and emotional symptoms, often described as postconcussion syndrome (PCS). The optimal recovery time including return-to-work (RTW) after mTBI is unclear. In this single-centre parallel-group trial, patients assigned three days (3D-group) or seven days (7D-group) sick leave were compared with a comprehensive neuropsychological test battery including the Post Concussion Symptom Scale (PCSS) within one week, after three and twelve months post-injury. The influence of the effective time until RTW on postconcussional symptoms and cognitive performance was analyzed. The 3D-group rated significantly higher mean scores in some PCSS symptoms, tended to fulfil diagnosis criteria of PCS more often and showed better cognitive performance in several neuropsychological test scores than the 7D-group at all three time-points of follow-up. Overall, patients returned to work 11.35 days post injury, thus distinctly above both recommended sick leaves. There was a trend for longer sick leave in patients randomized into the 3D-group. Further analyses revealed that the group with an absolute RTW within one week showed lower symptom severity in fatigue at three and twelve months, less PCS and faster performance in fine motor speed at twelve months than the group with an absolute RTW after one week. Our data underline the heterogeneity of mTBI and shows that acute and sub-acute symptoms are not prognostic factors for neuropsychological outcome at one year. Later ability to work seems to be prognostic for long-term occurrence of PCS.