Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES) data are used to derive the modal mineralogy of spectrally distinct Martian low-albedo regions and to identify spatial trends in mineralogic assemblages. Results from this work are consistent with the major results of previous spectroscopic studies: (1) Plagioclase and clinopyroxene are the dominant minerals of most southern highlands regions, (2) the northern plains exhibit the lowest pyroxene abundance within Martian low-albedo regions, and (3) the highest concentrations of high-silica phase(s) are found in the northern plains, Solis Planum and a few southern high-latitude regions. Low-albedo regions may be classified into four units on the basis of relative abundances of plagioclase, pyroxene, and high-silica phase(s). Unit distributions between ±45° latitude exhibit moderate correlation with distinct provinces (e.g., Syrtis Major, Aonium Sinus) defined by large-scale morphology, elevation, and to some extent, surface age, suggesting that the spectral and compositional differences between these units are more strongly controlled by original bedrock mineralogy than by surface-atmosphere interactions and alteration. Syrtis Major exhibits a difference in mineralogy from the surrounding highlands suggesting a differing degree of fractional crystallization, assimilation, or source region composition. Areas with thick crust near the Tharsis Plateau exhibit lower abundances of olivine and greater plagioclase/pyroxene ratios than surrounding highland terrains, suggesting that magmas in this region may have undergone increased olivine fractionation. Regions where surface alteration is more likely to be the primary control on observed spectral signatures are the high-latitude areas (>45°), where globally, surfaces dominated by high-silica phase(s) are most commonly found.