Protein-specific glycoform analysis is essential for the thorough understanding of cellular chemistry and signaling but presents a significant assay challenge for small-sized, free-floating exosomes, ubiquitous regulators of cellular physiological functions and mediators of intercellular communication. We report herein a quantitative localized analysis (QLA) method for the first-time achievement of a protein-specific glycosignature assay on these important extracellular vesicles. The integration of localized chemical remodeling and quantitative electrochemistry allows the proof-of-concept QLA examination of exosomal mucin 1 (MUC1)-specific terminal galactose/N-acetylgalactosamine (Gal/GalNAc). In combination with sialic acid (Sia) cleavage manipulation for the exposure of originally capped Gal/GalNAc, QLA has revealed distinct MUC1-specific sialylation capping ratios for MCF-7 and MDA-MB-231 exosomes, as well as when compared to parent cells. These findings suggest a useful noninvasive indicator for subtyping cancer cells and exosome secretion as a likely venue for the preservation of cellular compositional and functional integrity. The QLA method also permits dynamic monitoring of changes in the exosomal MUC1-specific sialylation capping ratio, enabling the distinction of biogenesis pathways of exosomes.