This article is about the complicated intersections of mental illness, diagnosis and narrative in life writing. It analyses challenges posed to the authority of diagnosis—both as medical label and mode of reading—within two memoirs about mental illness and celebrates the ensuing literary innovation in each text. As such, this article is situated as part of the continuing move within the critical medical humanities to develop more sophisticated readings of illness narratives and emphasises the importance of the role of literary studies to achieve this aim. Borrowing from and expanding Margaret Price’s concept of the counterdiagnostic as a tool that challenges a reader’s urge to explain, clarify and contain a narrator with mental disabilities, I will read Susanna Kaysen’s Girl, Interrupted (1993) and Lauren Slater’s Lying: A Metaphorical Memoir (2001) as two texts that challenge the organising structures of medical authority as they are manifested in diagnostic processes. In so doing I will reflect on the work of illness narratives and the force of the diagnostic moment, understood as a violent misreading of the expressions of mental illness in texts. My readings of these memoirs demonstrate how the material locations and political aesthetics of counterdiagnosis undermine the limited figuration of narrative offered by much work in narrative medicine, and deconstruct diagnosis, both in a medical and literary capacity. Counterdiagnosis is, then, posited as a crucial means of further opening up the analysis of illness narratives, specifically those of mental distress.