PurposeThe aim of this paper is to contribute to a greater understanding of non-profit organization (NPO) management control systems (MCS) and accountability in organizations providing support service for capacity constrained service users. Specifically, the paper examines the role of MCS and accountability in supporting mission realization in NPOs providing services to people with intellectual disabilities and reflects on this in the context of the COVID-19 pandemic.Design/methodology/approachThe research comprised a case study of four NPOs providing services to people with intellectual disabilities in Ireland conducted prior to the global COVID-19 pandemic. The study probed management's perceptions of stakeholders and examined the manner in which the design and use of MCS and accountability processes supported mission realization.FindingsService users were regarded as the least powerful stakeholder and consequently the least attended to in terms of MCS and accountability processes. The absence of relational and dialogical accountability with service users is not only central to maintaining this power asymmetry but also poses a threat to mission realization. These deficits can be addressed through the integration and monitoring of internal advocacy activities into MCS and accountability processes, which, on reflection, may also mitigate some of the negative consequences for service users of isolation from external support networks in times of crisis.Research limitations/implicationsThis research has opened up an area for enquiry – internal advocacy – heretofore not addressed in the management accounting literature, opening up a novel vein for future research. Such research could further examine the role of internal advocacy, drawing from and adding to the research in other support service domains. A number of objectives and questions might be considered: (1) probing the level of management recognition of the role of direct engagement in advocacy activities in supporting service user agency; (2) identifying with service users and management the nature and attributes of effective advocacy activities and practices; (3) questioning how such advocacy activities and practices might be reflected in MCS; (4) identifying what service user stakeholders regard as effective accountability to them in relation to their needs and objectives; and (5) assessing the impact on service user experience and on NPO mission realization of internal advocacy activities and the monitoring and review thereof through MCS. These suggestions for future research draw attention to aspects of support service delivery that have the potential to be profoundly influential on service outcomes.Practical implicationsA performance management model reflecting the identified need to incorporate internal advocacy mechanisms into organizational management control systems is proposed in an effort to increase accountability of NPOs to their core mission stakeholder – service users. This model may be of value to NPO management as they move from a medical-model of care to a rights-based model for service delivery in care settings.Social implicationsThe paper reflects the importance of listening to the voice of vulnerable service users in NPO care settings and proposes a mechanism for embedding internal advocacy in formal management control systems and accountability processes.Originality/valueIn proposing an “agency” supportive relational and dialogical accountability logic for such organizations, underpinned by “internal advocacy”, this research provides theoretical and practical insights for accountability processes and the design of MCS. The findings contribute empirically, not just to the NPO management and MCS literature but also to understanding the relational interaction of service users with service organizations, and what this means in supporting service user objectives and realization of organizational mission.