Magnesium-based metal matrix composites (Mg-based MMCs) are promising lightweight materials with many applications in the automotive and aerospace industries. Mg-based MMCs combine the high specific strength and good castability of Mg alloys with the excellent properties of the reinforcement. The reinforcing material is typically a ceramic, which exhibit tempting properties such as high melting point, excellent wear resistance, and high stiffness. The main driving force behind the development of Mg-based MMCs is that conventional Mg alloys show insufficient yield strength, damping capacity, wear, fatigue, thermal shock and creep resistances for many high-end applications, and thus, MMCs usually outperform their monolithic counterparts in this respect. However, it has been shown that Mg-based MMCs corrode faster than their monolithic counterparts. To date, attempts to produce corrosion-resistant Mg-based MMCs have failed, largely due to a lack of fundamental understanding of the corrosion phenomena occurring in this class of materials. In this paper, we studied the atmospheric corrosion behavior of silicon carbide (SiC)-reinforced Mg-based MMCs produced via a semi-solid casting method. We investigated the MMCs’ microstructure using the scanning transmission electron microscope (STEM/EDX) and electron energy loss spectroscopy (EELS). It was shown that the corrosion resistance of Mg-based MMCs can be improved via changing the casting parameters and pre-treatment of the reinforcements. A modified Mg-based MMCs exhibit favorable microstructural characteristics e.g., a lower fraction of pores and less numbers of ''undesirable'' intermetallic particles. We discuss, in detail, the atmospheric corrosion mechanism of Mg-based MMCs and formulate a working hypothesis for producing cast Mg-based MMCs that show enhanced atmospheric corrosion behavior.