The humanitarian sector has often been criticised for its hierarchical power dynamics. Such dynamics often centre the priorities of 'international' actors, thereby marginalising the knowledge and expertise of those closest to the setting and play out in various fora, including coordination mechanisms. While guidance emphasises the importance of supporting local systems and government structures rather than creating parallel humanitarian structures, this approach is not consistently applied, creating challenges. We used a case study approach to explore how power relations influence the practice of the Mental Health and Psychosocial Support Taskforce in Lebanon, a nationally-led coordination mechanism chaired by the Ministry of Public Health with UN agencies as co-chairs. We conducted 34 semi-structured interviews with Taskforce members and other stakeholders coordinating with the Taskforce, including local non-governmental organisations (NGOs), international NGOs, United Nations agencies and government ministries. Interview transcripts were collaboratively analysed using Dedoose. We conducted feedback workshops with participants and integrated their feedback into analysis. We found that UN agencies and international NGOs are perceived as holding more decision-making power due to their access to funding and credibility-both shaped by the humanitarian system. Our findings also suggest that power dynamics arising mainly from differences in seniority, relations between 'local' and 'expat' staff, and language used in meetings may affect, to varying degrees, decision-making power and members' voices. We also show how the agenda/focus of meetings, meeting format, language, and existing relationships with Taskforce leaders can influence levels of participation and decision-making in Taskforce meetings, ranging from lack of participation through being informed or consulted about decisions to decisions made in partnership. Our findings have broader implications for coordinating service delivery within the humanitarian sector, emphasising the need to reflect upon power imbalances critically and continually and to ensure a shared understanding of decision-making processes.