The Mental Capacity Act 2005 in England and Wales and Singapore’s Mental Capacity Act 2008 (which substantially transplants provisions from the former statute) might appear to be twins on paper, but they have gone on to lead very different lives. In this article, we examine how two broadly identical laws have taken on divergent identities within their respective jurisdictions when implemented and interpreted in the courtroom. We reveal and analyse differences in parliamentary intent concerning at what stage a person’s decision-making agency is putatively empowered; judicial development of central concepts; underlying socio-cultural commitments; and outline opportunities for bi-directional learning in mental capacity law across both jurisdictions.