Purpose Brain Injury Case Managers (BICMs) work closely with individuals with Acquired Brain Injury (ABI), assessing needs, structuring rehabilitation interventions and providing support, and have significant experience of clients with impairments to decision making. The purpose of this paper is to explore the application of the Mental Capacity Act (MCA) and its guidance when applied to ABI survivors. This research aimed to: first, highlight potential conflicts or tensions that application of the MCA might pose, and second, identify approaches to mitigate the problems of the MCA and capacity assessments with ABI survivors. It is hoped that this will support improvements in the services offered. Design/methodology/approach Using a mixed method approach, 93 BICMs responded to an online questionnaire about decision making following ABI. Of these, 12 BICMs agreed to take part in a follow-up semi-structured telephone interview. Findings The data revealed four main themes: disagreements with other professionals, hidden disabilities, vulnerability in the community and implementation of the MCA and capacity assessments. Practical implications The findings highlight the need for changes to the way mental capacity assessments are conducted and the need for training for professionals in the hidden effects of ABI. Originality/value Limited research exists on potential limitations of the application of the MCA for individuals with an ABI. This paper provides much needed research on the difficulties surrounding mental capacity and ABI.