Introduction: People seeking asylum are at increased risk of mental health difficulties due to pre- and post-migration experiences. This review synthesised the findings of studies investigating asylum seekers’ experiences of the asylum determination process in the EU+ and UK. The review sought to understand how these procedures influence asylum seekers mental health.Methods: The Web of Science, MEDLINE, PsycINFO, EBSCO, and Scopus were searched from inception to 24 August 2023. Peer reviewed studies were eligible if they provided a qualitative analysis of primary data from adult asylum seekers and refugees regarding lived experience of asylum determination procedures in the UK and EU+. Articles reporting on contextual factors associated with the mental health and well-being of adult asylum seekers were included. Studies with only a subset of relevant participants were eligible if data were reported separately. Studies with a majority of refused asylum seekers were excluded unless data was labelled distinctly. The protocol was not pre-registered.Findings: From a total of 4902, 45 studies were included in the qualitative thematic synthesis, representing the perspectives of 1158 asylum seekers and refugees: male (n= 506), female (n=394) and not reported (n= 258, k=8), and age range [17 to 88 years: k =30].The themes demonstrate high levels of psychological distress during and after the asylum process. This was associated with the hostile environments created by policies and procedures. Participants’ mental health was also affected by being caught in a stalemate while awaiting the outcome of their claim. The procedures exacerbated previous mental health difficulties. External and internal protective factors were identified, such as NGOs, religion, and cognitive strategies.Interpretation: The accumulation of each step of the asylum process contributes to psychological distress and exacerbates mental health difficulties, contributing to longer-term consequences for asylum seekers and refugees. Preventative strategies and policy changes are recommended.