This article draws upon the civic republican tradition to offer new conceptual resources for the normative assessment of mental capacity law. The republican conception of liberty as non-domination is used to identify ways in which such laws generate arbitrary power that can underpin relationships of servility and insecurity. It also shows how non-domination provides a basis for critiquing legal tests of decision-making that rely upon ‘diagnostic’ rather than ‘functional’ criteria. In response, two main civic republican strategies are recommended for securing freedom in the context of the legal regulation of psychological disability: self-authorisation techniques and participatory shaping of power. The result is a series of proposals for the reform of decisional capacity law, including a transition towards purely functional assessment of decisional capacity, surer legal footing for advanced care planning, and greater control over the design and administration of decision-making capacity laws by those with psychological disabilities.