This paper seeks to trace the origins of offences by youths as a distinct social concern in Lagos and examines the categorization of a group, the ‘juvenile delinquent’, by colonial administrators and welfare officers. While organized pickpocketing and prostitution by young people emerged as an issue in Nigerian newspapers in the 1920s, it was largely ignored by local administrators until the appointment, in 1941, of the first Social Welfare Officer. This led to the implementation of new administrative and judiciary machinery which combined two processes: it legislated ‘juvenile delinquency’ into existence as a clearly identifiable social problem; and criminalized a large portion of urban youth, especially female hawkers. The combination of these processes constitutes what can be called the invention of juvenile delinquency in Nigeria.