The implementation of corporate social responsibility (CSR) is crucial for the legitimacy of an organization in today’s globalized economy. This study aims to enrich our knowledge of the implementation of the largest voluntary CSR initiative—the UN Global Compact (UNGC). Drawing on insights from stakeholder, network, and institutional theory, I derive a positive impact of UNGC participation duration on the implementation level of the UNGC principles, despite potential weaknesses in the initiative’s accountability structure. Moreover, I scrutinize the validity of the newly introduced UNGC “Differentiation Programme” before applying this framework in the empirical analysis. Results from ordinal, linear, and instrumental variable regression models suggest that, contrary to claims made by UNGC critics, the duration of UNGC participation does affect the level of UNGC implementation. However, this effect appears to be much smaller than previous practitioner studies have suggested. Moreover, strong local UNGC networks affect the implementation level of the UNGC positively. Their hypothesized moderating role between UNGC participation duration and UNGC implementation level, however, is only significant in networks with activities of high quality rather than high quantity.