The grain refinement of Mg–Al alloy AZ91 via carbon inoculation, including the significant role of Mn in advanced nucleation, was analyzed, and the corresponding mechanical properties and aging behavior were investigated. To this end, various amounts of C were added into the liquid at the desired temperatures. Al8Mn5 particles, which are suitable nucleation sites for α-Mg, were identified as the primary grain refiners. In situ particle formation, along with appropriate wetting and a suitable orientation relationship (OR), facilitated the grain refinement mechanism. Al4C3 particles contributed to heterogeneous nucleation by providing suitable Al8Mn5 nucleation sites. Mn removal resulted in poor grain refinement in the Mg–Al alloy. The Hall–Petch relationship, high-temperature tensile behavior, and aging mechanism of the samples refined by 1 wt % C addition (as the best grain refiner) are discussed and compared with industrial practice.

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