Mycobacterium is a diverse genus of actinobacteria that includes the causative agents of human tuberculosis and leprosy. Mycobacteria are protected by their unique, multilaminar cell envelope, which grants them intrinsic resistance to environmental challenges such as antibiotics. This essential cellular structure is elongated at the polar ends of cells, but the regulation of cytosolic precursor synthesis and localized envelope synthesis remains unclear. Here, we present the PMf (plasma membrane free of cell wall components), a membrane domain distinct from the bulk plasma membrane of Mycobacterium smegmatis. Proteomic and lipidomic characterization demonstrate that the PMf contains lipidic substrates of cell envelope synthesis as well as the essential enzymes that produce these lipids. Live imaging of cells expressing PMf-targeted fluorescent reporters reveals that the PMf is enriched near the site of new cell envelope synthesis and is present upon the formation of a new growth pole. Additionally, we found that during nutrient starvation and antibiotic treatment, the PMf redistributes from the cell poles to the sidewall of the cell body, suggesting its role in general cell envelope maintenance. These observations support a model in which the PMf functions to spatiotemporally coordinate cell elongation by providing the necessary precursor molecules at the site of envelope synthesis.