A proper understanding of the historical societal metabolism of cobalt (Co) in the European Union (EU) is lacking, resulting in a serious weakness in identifying potential sources of secondary materials, enhancing circularity, and improving forecasting assessments. A retrospective dynamic material flow analysis was performed to assess the stocks and flows of Co in the EU27, considering a number of Co-containing commodities and final metal products. Detailed results are provided for the year 2018, and the evolution of the stocks and flows over time is presented from 1955 to 2018. The results for 2018 indicate that the largest stock in the EU was the landfill stock, with around 166,000 metric tons of Co (35% of the total stock in that year), followed by the in-use stock, with around 157,000 metric tons of Co (33% of the total stock). From 1970 onwards, the in-use stock has doubled its size every 10 or 11 years and the landfill stock every eight years. A sensitivity analysis was performed with eight sets of parameters to determine their impact on the stocks in 2018. Six of them have a marginal impact on the stocks. The in-use stock and the stock of losses are mainly affected by the data uncertainty related to the end-use demand of Co and the lifetimes of the applications (between 4 and 22% in absolute terms for the in-use stock, and between 7 and 27% in absolute terms for the stock of losses).