Open access. Mental health legislation in Northern Ireland has always been separate from legislation in the rest of the UK; the Mental Health (Northern Ireland) Order (MHO) had been in place since 1986. In common with other jurisdictions, this legislation utilises the presence of mental disorder and risk as criteria for detention and involuntary treatment. The MHO has been replaced by the Mental Capacity Act (Northern Ireland) 2016 (MCA), an example of ‘fusion’ legislation in which impairment of decision-making capacity and best interests are the only criteria to be used when making decisions across health and social care. In this paper, we outline the development of the MCA to date, and discuss its potential to improve mental healthcare by placing the treatment of mental illness within the same legislative framework as physical illnesses.