To systematically investigate the precipitation and evolution mechanisms of MnS during the solidification of medium-carbon low-alloy steel, in-situ observation experiments using high temperature confocal laser scanning microscopy (HT-CLSM) were conducted. The effect of cooling rate (i.e., 50, 150, and 300 °C/min) in the solidification region on the dynamic precipitation behavior of MnS was performed. The obtained results show that the MnS precipitates mainly exhibit three typical morphologies, i.e., angular, globular, and dendritic after solidification. With the increasing cooling rate, the precipitation starting temperature (Tp,s) of dendritic MnS decreases from 1290.5 to 1235.7 °C, while the finishing temperature (Tp,f) increases from 1053.3 to 1135.6 °C, resulting in a shorter precipitation time. Meanwhile, higher cooling rates can suppress the growth kinetics of MnS, leading to a significant reduction in the maximum inclusion size from 143.3 to 37.3 μm, thereby achieving a pronounced grain refinement. In addition, the increased cooling rate can enhance the microsegregation of Mn and S elements, intensifies the supersaturation level and nucleation driving force of MnS, and consequently promotes an increase of MnS number density during the final stage of solidification. The obtained findings reveal the precipitation kinetics and morphology evolution mechanism of MnS during the solidification range of 1550 to 1400 °C, providing an important theoretical guidance for controlling sulfide precipitates during solidification as well as contribute to the “inclusion/precipitate engineering” concept.