The brazeability of AZ31B-H24 magnesium alloy sheet to Sn-coated plain carbon steel sheet using a Mg-AI-Zn alloy filler metal and a diode laser heat source has been investigated. While the Sn coating promoted good wetting between the molten filler metal and the steel sheet, it did not play a role in forming the final metallic bond. Its primary function appeared to be in maintaining an oxide-free steel surface until the molten Mg filler metal could come in direct contact with the steel surface. In all cases, failure of transverse tensile shear test specimens of the joint occurred in the steel base metal. Metallic bonding between the magnesium alloy and the steel was facilitated by the formation of two transition layers, including a Fe(AI) solid solution formed on the surface of the steel and a nano-scale layer of Al8(Mn, Fe)5 phase on the Fe(AI) surface layer. Examination of the Fe(AI)-AI8(Mn, Fe)5 and Al8(Mn, Fe)5-Mg interfaces using HR-TEM showed that orientation relationships (OR) with a low angle of rotation of the matching planes and low interplanar mismatch and therefore low interfacial energy density existed at the Fe(AI) AI8(Mn, Fe)5 interface (i.e., when [101¯1]Al8Mn5//[1¯11]Fe(Al),{110}Fe(Al) was 4.2 deg from {303¯3}Al8Mn5 with 5.2% interplanar mismatch) and a high angle of rotation of the matching planes and large interplanar mismatch, and therefore, high interfacial energy density existed at the Al8Mn5-Mg interface (i.e., when [101¯1]Al8Mn5//[101¯1]Mg, {303¯3}Al8Mn5 was within 47.4 deg of the {0002}Mg with 16.8% interplanar mismatch). These results were further validated using an edge-to-edge crystallographic matching model of the Fe(AI)-Al8(Mn, Fe)5 and Al8(Mn, Fe)5-Mg interfaces.