This thesis examines the ways in which the British mandate conceptualised, encountered, and sought to manage mental illness in Palestine between 1920 and 1948. The subject of mental illness has hitherto received partial consideration by historians interested in the Yishuv, who treat this period as formative for the Israeli mental health service. This thesis shifts the focus from European Jewish psychiatrists to the British mandate’s engagements with mental illness, thus contributing to the well-developed literature on colonial psychiatry. Where this thesis departs from many of these institutionally-focussed histories of colonial psychiatry is in its source base; lacking hospital case files or articles in psychiatric journals, this thesis draws on an eclectic range of material from census reports and folklore research to petitions and prison records. In bringing together these strands of the story of psychiatry and mental illness, this thesis seeks to move beyond the continued emphasis in the historiography of Palestine on politics, nationalism, and state-building, and to develop our understanding of state and society by examining how they interacted in relation to the question of mental illness. This thesis thus widens the cast of historical actors from psychiatric experts alone to take in policemen, census officials, and families. In addition, this thesis seeks to situate Palestine within wider mandatory, British imperial, and global contexts, not to elide specificities, but to resist a persistent historiographical tendency to treat Palestine as exceptional. The first part traces the development of British mandatory conceptualisations of mental illness through the census of 1931 and then through a focus on specific causes of mental illness thought to be at work in Palestine. The second part examines two contexts in which the mandate was brought into contact with the mentally ill: the law and petitions. The final part of the thesis explores two distinct therapeutic regimes introduced in this period: patient work and somatic treatments.