The hot deformation of cast TXA321 alloy has been studied in the temperature range 300-500 °C and in the strain rate range 0.0003-10 s ^-1 by developing a processing map. The map exhibited four domains in the temperature and strain rate ranges: (1) 300-325 °C and 0.0003-0.001 s^-1, (2) 325-430 °C and 0.001-0.04 s^-1, (3) 430-500 °C and 0.01-0.5 s^-1, and (4) 430-500 °C and 0.0003-0.002 s^-1. The first three domains represent dynamic recrystallization, resulting in finer grain sizes in the first two domains and coarser in the third domain. In the fourth domain, the alloy exhibited grain boundary sliding resulting in intercrystalline cracking in tension and is not useful for its hot working. Two regimes of flow instability were identified at higher strain rates, one at temperatures <380 °C and the other at >480 °C. The hot workability of Mg-3Sn-2Ca-1Al alloy has been established through the development of its processing map based on dynamic materials model. Domains 1-3 are considered as desirable processing windows for forming this alloy due to extensive dynamic recrystallization and grain refinement. Domain 4 represents extensive grain boundary sliding, and should be avoided under stresses of tensile nature due to flow instability.