During humanitarian crises, under-resourced and overstretched health systems may not be able to fully meet mental health and psychosocial support (MHPSS) needs of affected populations, including refugees, internally displaced persons and host communities. Health system governance is vital to humanitarian health response, but there has been little research on this, particularly for MHPSS. We present a case study of a national MHPSS coordination mechanism (the MHPSS Taskforce) in Lebanon, a country which has experienced multiple crises and hosts over 2 million refugees. The aim was to explore the barriers and enablers facing the MHPSS Taskforce in responding to the needs of displaced and host populations in Lebanon. Interviews were conducted with 34 key stakeholders, including Taskforce members, representatives from non-governmental organisations, United Nations agencies, and government Ministries. Our findings show that the positioning of MHPSS within the humanitarian cluster system acts as a barrier to mounting an effective response, with the MHPSS Taskforce sometimes siloed rather than integrated across clusters. Coordination within the Taskforce was reported to be effective in some respects, but limited by a lack of clarity about its decision-making processes, affiliation, mandate, and inclusion of regional perspectives and key groups such as mental health services users in Lebanon. While the technical capacity of the Taskforce is strong, limited funding and staffing were seen to impact its capacity to effectively oversee the MHPSS response in Lebanon. Key recommendations include: the need for stronger mechanisms and operating procedures for interagency and inter-sectoral collaboration on MHPSS within the humanitarian cluster system; greater clarity on the role of the Taskforce and key Taskforce actors, streamlined reporting channels and greater inclusion of diverse perspectives, particularly mental health service users; and greater financial and human resources within coordination mechanisms to support the national MHPSS response in Lebanon.