The use of hydropower provides an important technical advancement over hand-operated grain mills, which steadily increased over large parts of Europe from the early Middle Ages onwards. Since written information on the technical design of early medieval water mills is generally missing, archaeological evidence may provide unique insights into their evolution. Well-preserved wooden finds from continental Europe are, however, extremely rare. Here we present dendroarchaeological results from an exceptional number of structural elements of the Audun-le-Tiche water mill in northern France. Taxonomical identification, tree-ring dating and observations of technical features provide a detailed picture of milling technology as early as the Carolingian period in the mid-9th century. A well-preserved waterwheel segment allows the reconstruction of an undershot start-and-float wheel. Numerous wooden paddles reveal a technological evolution from one-piece paddles to composite forms. Placing our results in the context of other early medieval mills, suggests a rather uniform consruction design within, though different beyond the Frankish Empire. This study provides a detailed description of early medieval water milling technology that possibly contributed to the success of agriculture as well as cultural and economic growth of the Carolingian Empire.