Although ‘modern’ mental health care comprises a variety of theoretical approaches and practices, the supposed identification of ‘mental illness’ can be understood as being made on the basis of a specific conception of subjectivity that is characteristic of ‘modernity’. This is to say that any perceived ‘deviation’ from this characteristically ‘modern self’ is seen as a possible ‘sign’ of ‘mental illness ’, given a ‘negative determination’, and conceptualized in terms of a ‘deficiency’ or a ‘lack’; accordingly, the ‘ideal’ ‘therapeutic’ aim of ‘modern’ mental health care can be understood as the ‘rectification’ of that ‘deficiency’ through a ‘re-instatement’ of the ‘modern self’. Although contemporary mental health care is increasingly becoming influenced by the so-called ‘death’ of the ‘modern self’, this paper will suggest that it is the work of the 20th century French philosopher, Gilles Deleuze, that is able to provide mental health care with a coherent determination of a ‘ postmodern self’. However, a Deleuzian account of subjectivity stands in stark contrast to ‘modernity’s’ conception of subjectivity and, as such, this paper will attempt to show how this ‘ postmodern’ subjectivity challenges many of the assumptions of ‘modern’ mental health care. Moreover, acknowledging the complexity and the perceived difficulty of Deleuze’s work, this paper will provide an account of subjectivity that can be understood as ‘Deleuzian’ in its orientation, rather than ‘Deleuze’s theory of subjectivity’, and therefore, this paper also seeks to stimulate further research and discussion of Deleuze’s work on subjectivity, and how that work may be able to inform, and possibly even reform, the theoretical foundations and associated diagnostic and therapeutic practices of psychiatry, psychotherapy, and mental health nursing.