Over the twentieth century, the Lunacy Office (renamed the Court of Protection in 1947) was responsible for appointing ‘receivers’ to manage the property of adults in England who were found incapable of managing their own affairs. Tens of thousands of people were in this position by the 1920s, and numbers continued to grow until after Second World War. This article uses the archives of the Office to examine the evolution of the concept of mental incapacity over the first half of the twentieth century, offering a corrective to the popular impression that the time before the Mental Capacity Act of 2005 was an era of ignorance and bad practice. It examines the changing ways in which being ‘incapable’ was understood and described, with particular reference to shifting ideas of citizenship. I argue that incapacity was not always seen as absolute or permanent in the first half of the century, that models of incapacity began to include perceived vulnerability in the interwar period and that women in particular were seen in this way. From the 1940s, though, the profile of those found incapable was changing, and the growing welfare state and its principles of employment and universality saw the idea of incapacity narrowing and solidifying around knowledge deficits, especially among the elderly. This brings the history of the Lunacy Office into the twentieth century and connects it to current concerns around assessments of mental capacity today.