This article presents a diachronic survey of forms and functions of the subjunctive II grammatical category in historical German. The subjunctive II is historically based on the subjunctive preterit form but has lost its temporal function and become a purely modal category. It has developed a variety of synthetic and analytical forms since Old High German, especially in vernacular varieties, but also in Standard German, and fulfils a range of functions, chiefly that of non-affirmativity. The present study aims to shed light on the historical development of the subjunctive II’s form and function. Its focus is on East Upper German, and its temporal scope spans over a millennium from the 9th to the 19th century. In order to study the usage and development of the subjunctive II in historical East Upper German sermons, the south-eastern part of SermonC, a diachronic corpus of historical German sermons, is analysed. While this (sub‑)corpus is restricted in terms of region and genre, it provides a unique window to written language usage, variation and change in a clearly defined domain over a very long time. The results show that the refunctionalization of the subjunctive preterit as a purely modal category does not date to prehistoric times, as is generally assumed; instead, they point to the 11th century as the likely time for this change, with limited temporal uses persisting into the 15th century. The 15th century is also the time when the synthetic subjunctive II form with -et/-at, a typical feature of Bavarian dialects, seems to have emerged in the spoken register. Analytical forms started to appear from the 13th century, and the periphrasis with würde, today accepted in Standard German, prevailed among several written German analytical variants in the 18th century. While Standard German has preserved the distinction between two subjunctive categories, East Upper German spoken language has in effect reduced a two-way morphological system (tense and mood) to a pure mood system, with tense expressed syntactically, in effect refunctionalizing morphological tense as a mood marker.