Abstract Purpose Early intervention can improve mental health outcomes for people living with serious mental illness (SMI). Understanding what factors influence patients’ health help-seeking decisions are important in low and middle-income countries (LMICs) where resources and outcomes are poor, particularly in slums, to help inform targeted intervention approaches. Methods A concurrent triangulation mixed methods study conducted in Dhaka, Bangladesh, using a quantitative pathway to care questionnaire with individuals from a local slum (Korail) attending the National Institute for Mental Health (NIMH), a specialised hospital for mental health services. Qualitative interviews were conducted with people with SMI and family caregivers living in Korail. Results 28,896 patients attended NIMH between 24th September 2022 and 25th September 2023 and only 0.11% ( n = 33) came from the Korail slum. 46% had previously seen a faith or traditional healer. Qualitative interviews with people with SMI and caregivers in Korail showed that spiritual possession was among several perceived causes of SMI. Participants also percieved pharmacological treatment as expensive and potentially harmful. However participants also reported a lack of knowledge about specialist mental health facilities and spending considerable funds and resources on healers and private medical providers in the hope of cure. Conclusions Help-seeking among families living in slums in Dhaka is pluralistic, with complex influences on treatment choice. Understanding help-seeking behaviour and care pathways is crucial to design an equitable health system and improve access to effective mental health care.