Background: Scalable psychological interventions such as the WHO’s Self-Help Plus (SH+) have been developed for clinical and non-clinical populations in need of psychological support. SH+ has been successfully implemented to prevent common mental disorders among asylum seekers and refugees who are growing in number due to increasing levels of forced migration. These populations are often exposed to multiple, severe sources of traumatisation, and evidence of the effect of such events on treatment is insufficient, especially for non-clinical populations. Objective: We aim to study the effect of potentially traumatic experiences (PTEs) and the mediating role of symptoms of posttraumatic stress disorder (PTSD) on the improvement following SH+. Method: Participants allocated to SH+ who received at least three sessions (N = 345) were extracted from two large, randomised, European prevention trials involving asylum seekers and refugees. Measures of distress, depression, functional impairment, and post-traumatic stress symptoms were administered at baseline and 6 months post-intervention, together with measures of well-being and quality of life. Adjusted models were constructed to examine the effect of PTEs on post-intervention improvement. The possible mediating role of PTSD symptoms in this relationship was then tested. Results: Increasing numbers of PTEs decreased the beneficial effect of SH+ for all measures. This relationship was mediated by symptoms of PTSD when analysing measures of well-being and quality of life. However, this did not apply for measures of mental health problems. Conclusions: Exposure to PTEs may largely reduce benefits from SH+. PTSD symptomatology plays a specific, mediating role on psychological well-being and quality of life of participants who experienced PTE. Healthcare professionals and researchers should consider the role of PTEs and PTSD symptoms in the treatment of migrants and refugees and explore possible feasible add-on solutions for cases exposed to multiple PTEs.