British laws which sought to control and prevent street prostitution in the early twentieth century all relied on the idea that a ‘common prostitute’ was a legally definable person, and, while prostitution itself was not an offence, that the action of street solicitation represented a special kind of public nuisance. This article explores some of the implications of this legal system, especially after prostitutes were added to the fingerprinting schedule of the London Metropolitan Police in 1917. Centred around one rare case-file concerning the mistaken identity of a street prostitute in 1920, the article explores the way in which women working as prostitutes experienced and negotiated the criminal justice system. In contrast to the historical attention given to the Contagious Diseases Acts, the solicitation laws are seriously under-examined. Yet these laws were put in place prior to the CD Acts, lasted long after their repeal, affected a far greater number of women, and were significantly more important to the police and the state in their control of prostitution than were the short-lived and geographically limited CD Acts. In the context of the CD Acts, historians have looked at the ways in which a prostitute identity was developed and assigned by medical discourse and medical registration. However, the far more common and long-lasting experience of prostitute women in Britain was governed by the solicitation laws and a legal, not medical, process of classification. Through Nellie Johnson’s story, we can begin to explore the intricacies of a legal system of prostitution control peculiar to Britain at a crucial point in its development. This article argues that over the course of the early twentieth century, the criminalization of identity became the grounds upon which the entire system of street- prostitution control in England and Wales rested. The fingerprinting of prostitutes, and Nellie Johnson’s personal experiences, fit into a larger story of modernization in early twentieth-century Britain and the early twentieth-century world. This period witnessed the development of particular, and technical, forms of identification which were applied to particular groups of people, an abstraction which turned the body itself into a text that had very real consequences for women like Nellie Johnson.