This article argues that governmentality perspectives provide a fruitful way to analyse the complex interactions between disability and the law, using Article 12 of the Convention on the Rights of Persons with Disabilities (CRPD) and mental disability as a ‘case study’. The governmentality literature recognises that the law is implicated in the truths, knowledges, rationales and practices through which disability is rendered known and knowable. From this perspective, a governmentality approach provides a framework for considering the complex and contradictory relationships between capacity, disability and law. This article analyses the concept of legal capacity in Article 12 and its challenge to mental capacity as a central principle in developed Western legal systems. The article shows that rights claims in the second half of the twentieth century reinforced the classical privileging of cognitive ability and rational capacity as a threshold requirement for citizenship. As a consequence, capacity and incapacity operate as mutually constitutive dialogues of normality, in which determinations of incapacity disqualify individuals with mental disabilities from mainstream social discourse and from recognition before the law. Paradoxically, the rules of exception operate differently in the criminal law. On one hand the criminal law recognises that mental disability diminishes criminal culpability. On the other, mental disability frequently goes unrecognised in the day-to-day conduct of the criminal justice system and in the adjudication of minor offences, resulting in the hyper-incarceration of those with mental disability. This article argues that the radical re-conceptualisation of the notion of legal capacity in the CRPD promises to recast decision-making relationships throughout the legal system by replacing determinations of capacity and incapacity with strategies of support. The article observes that the radical agenda may be undermined if the rationale and consequences of this shift are not fully appreciated.