High-resolution capillary zone electrophoresis - mass spectrometry (CZE-MS) has been of increasing interest for the analysis of biopharmaceuticals. In this work, a combination of middle-down and intact CZE-MS analyses has been implemented for the characterization of a biotherapeutic monoclonal antibody (mAb) with a variety of post-translational modifications (PTMs) and glycosylation structures. Middle-down and intact CZE separations were performed in an acidified methanol-water background electrolyte on a capillary with a positively charged coating (M7C4I) coupled to an Orbitrap mass spectrometer using a commercial sheathless interface (CESI). Middle-down analysis of the IdeS-digested mAb provided characterization of PTMs of digestion fragments. High resolution CZE enabled separation of charge variants corresponding to 2X-deamidated, 1X-deamidated, and non-deamidated forms at baseline resolution. In the course of the middle-down CZE-MS analysis, separation of glycoforms of the F-C/2 fragment was accomplished due to hydrodynamic volume differences. Several identified PTMs were confirmed by CZE-MS2. Incorporation of TCEP-HCl reducing agent in the sample solvent resulted in successful analysis of reduced forms without the need for alkylation. CZE-MS studies on the intact mAb under denaturing conditions enabled baseline separation of the 2X-glycosylated, 1X-glycosylated, and aglycosylated populations as a result of hydrodynamic volume differences. The presence of a trace quantity of dissociated light chain was also detected in the intact protein analysis. Characterization of the mAb under native conditions verified identifications achieved via intact analysis and allowed for quantitative confirmation of proteoforms. Analysis of mAbs using CZE-MS represents a complementary approach to the more conventional liquid-chromatography - mass spectrometry-based approaches.