Common mental health disorders (CMDs) disproportionately affect people experiencing socioeconomic disadvantage. Non-pharmaceutical interventions, such as ‘social prescribing’ and new models of care and clinical practice, are becoming increasingly prevalent in primary care. However, little is known about how these interventions work and their impact on socioeconomic inequalities in health. Focusing on people experiencing socioeconomic disadvantage, this systematic review aims to: (1) explore the mechanisms by which non-pharmaceutical primary care interventions impact CMD-related health outcomes and inequalities; (2) identify the barriers to, and facilitators of, their implementation in primary care. This study is a systematic review of qualitative studies. Six bibliographic databases were searched (Medline, ASSIA, CINAHL, Embase, PsycInfo and Scopus) and additional grey literature sources were screened. The included studies were thematically analysed. Twenty-two studies were included, and three themes were identified: (1) agency; (2) social connections; (3) socioeconomic environment. The interventions were experienced as being positive for mental health when people felt a sense of agency and social connection. The barriers to effectiveness and engagement included socioeconomic deprivation and underfunding of community sector organisations. If non-pharmaceutical primary care interventions for CMDs are to avoid widening health inequalities, key socioeconomic barriers to their accessibility and implementation must be addressed.