In low- and middle-income countries like Sierra Leone, there is a significant gap in the treatment of perinatal mental health disorders such as anxiety, depression and somatization. This study explored the feasibility, acceptability and preliminary effectiveness of a culturally adapted Problem-Solving Therapy – Friendship Bench Intervention (PST-FBI) delivered by nonspecialists, mother-to-mother support groups (MMSGs), to perinatal women experiencing psychological distress. MMSGs provide 4 weeks of home-based, individual PST-FBI, followed by a peer-led group session called col at sacul (circle of serenity). The intervention targeted peri-urban pregnant women and new mothers screened for psychological distress. This was a two-armed, pre–post, waitlist-controlled study that employed the Sierra Leone Perinatal Psychological Distress Scale (SLPPDS) to screen and measure their outcomes. Feasibility and acceptability were examined through in-depth interviews using the Consolidated Framework for Implementation Research, analyzed thematically, while preliminary effectiveness was evaluated with chi-squared analysis for categorical and t-test for continuous variables. Twenty of the 25 women completed all four PST-FBI sessions delivered by five MMSGs. The individual PST and the peer-led session were viewed as beneficial for problem-sharing and skill building. The SLPPDS scores significantly dropped by 58.9% (17.1–8.4) in the intervention group, while the control group showed a 31.6% (18.0–12.3) decrease. The intervention’s effect size was d = 0.40 (p < 0.05). The MMSG-led PST-FBI, including the col at sacul session, proved feasible, acceptable and with preliminary effectiveness in improving the mental health of peri-urban pregnant women and new mothers in Sierra Leone. Further randomized-controlled trials are recommended before nationwide implementation.