Welcome to MPLR 2023, the 20th International Conference on Managed Programming Languages and Runtimes, held in Cacais, Portugal on Sunday 22 October 2023, co-located with SPLASH 2023. MPLR is a successor to the conference series on Managed Languages and Runtimes (ManLang). It is a premier forum for presenting and discussing novel results in all aspects of managed programming languages and runtime systems, which serve as building blocks for some of the most important computing systems around, ranging from small-scale (embedded and real-time systems) to large-scale (cloud-computing and big-data platforms) and anything in between (mobile, IoT, and wearable applications).