In the development of third-generation advanced high-strength steels, achieving a balance between strength and ductility while minimizing alloying and production costs is critical. Among the promising candidates, medium Mn steels (MMnS) have the desired design flexibility for achieving a certain amount of metastable austenite, thereby exhibiting an enhanced transformation-induced plasticity (TRIP) effect. Owing to the weakened alloying effect in low-Mn content MMnS, more efforts should be devoted to enhancing the stability of austenite during intercritical annealing. This study explores the possibility of developing high-strength, high-ductile MMnS via continuous cooling from medium temperatures. A 0.2C-3Mn-1.5Si (mass fraction, %) MMnS was selected to analyze the influence of Mn pre-partitioning on bainite transformation and the volume fraction of retained austenite in low-Mn content MMnS using various characterization methods, including SEM and EBSD, as well as mechanical property testing methods. The results indicate that the carbide-free bainite transformation occurring during the medium-temperature continuous cooling enables the acquisition of retained austenite in MMnS. Mn-rich austenite lamellae can be initially produced via Mn pre-partition during intercritical annealing. Subsequently, bainite transformation is restricted to occur within the undercooled austenite lamellae in the medium-temperature continuous cooling process, resulting in a refined multiphase microstructure comprising film-like retained austenite, bainitic ferrite, and intercritical ferrite. The volume fraction of retained austenite in low-Mn content MMnS substantially increases because of the enrichments of Mn and C from Mn pre-partition and bainite transformation, respectively, thereby enhancing the strength and ductility of MMnS.