While the unit cell volume of compounds belonging to the Mn₃Ga_1-xSn_xC (0 ≤ x ≤ 1) series shows a conformity with Vegard’s law, their magnetic and magnetocaloric properties behave differently from those of parent compounds Mn₃GaC and Mn₃SnC. A correlation between the observed magnetic properties and underlying magnetic and local structure suggests that replacing Ga atoms by larger atoms of Sn results in the formation of Ga-rich and Sn-rich clusters. As a result, even though the long range structure appears to be cubic, Mn atoms find themselves in two different local environments. The packing of these two different local structures into a single global structure induces tensile/compressive strains on the Mn₆C functional unit and is responsible for the observed magnetic properties across the entire solid solution range.