In this paper, I propose an existentialist-phenomenological model that conceives of mental illness through the terminology of Heidegger's Being and Time. In particular, the concepts of (i) existentiality, (ii) disturbance and (iii) the relation between "being-with' and "the one', will be implemented in order to reconstruct the experience of mental illness. The proposed model understands mental illness as a disturbance of a person's existentiality. More precisely, mental illness is conceptualized as the disturbance of a person's existential structure, the process of which leads to a becoming explicit of the otherwise implicit dynamical structure that constitutes a person's experience. In particular, the existential component of "being-with' comes to play a central role in the disturbance of existentiality, thus, I will claim, that it enables a person's structure of experience to be "open for normativity'. By adopting a pragmatist stance on Heideggerian phenomenology, the suggested model proves compatible with naturalist and normativist theories of mental illness while still offering a phenomenological description of the phenomenon.