The number of adults who self-neglect and thus fall under the aegis of local authority adult safeguarding procedures in England has increased substantially since the introduction of the Care Act 2014. The requirement for collaborative working between agencies dealing with these adults in a safeguarding context is explicit in government policy and legislation. Decisions made by the multiplicity of agencies that may work with people who self-neglect are largely guided by the Mental Capacity Act 2005 (MCA). The overall objective of this research was to develop a clearer understanding of how the range of agencies that might typically be involved in the life of a self-neglecting person work together. This article examines how agencies put the MCA into practice in their work with people who self-neglect, and how they understand their own and others' roles and responsibilities in so doing. This qualitative study took place in two local authorities in England from 2016 to 2017 and informed a wider action research study which was completed in 2019. Non-probability purposive sampling was used to recruit participants from the professional groups who might typically be involved with self-neglect cases. À total of 245 participants from across 17 different professional groups took part in semi-structured interviews, in a group, paired or individual format, decided by their customary working configuration. Data from the interview transcripts was analysed using thematic analysis. Three key themes in relation to how participants understood the MCA and multi-agency working emerged from the analysis of this data set. These were; a lack of understanding of the MCA by participants and other agencies; a reluctance to engage with MCA assessments; and a perception of manipulation of the MCA by other professionals. This study underlines the importance of the informed application of the MCA in working with people who self-neglect, and an urgent need to consider how this could be enhanced if the service user is not to experience intrusive interventions resulting from professional misinterpretation.