Despite growing numbers of unaccompanied refugee minors (UMs) in Europe, and evidence that this group is at risk of developing mental health problems, there still remain important knowledge gaps regarding the development of UMs’ mental health during their trajectories in the host country and, in particular, the possible influencing role of traumatic experiences and daily stressors therein. This study therefore followed 103 UMs from the moment they arrived in Belgium until 18 months later. Traumatic experiences (SLE), mental health symptoms (HSCL-37A, RATS) and daily stressors (DSSYR) were measured at arrival in Belgium, after 6 and 18 months. UMs reported generally high scores on anxiety, depression and post-traumatic stress disorder (PTSD). Linear mixed model analysis showed no significant differences in mental health scores over time, pointing towards the possible long-term persistence of mental health problems in this population. The number of traumatic experiences and the number of daily stressors leaded to a significant higher symptom level of depression (daily stressors), anxiety and PTSD (traumatic experiences and daily stressors). European migration policies need to reduce the impact of daily stressors on UMs’ mental health by ameliorating the reception and care facilities for this group. Moreover, regular mental health screenings are needed, in combination with, if needed, adapted psychosocial and therapeutic care.