Monocyte extravasation through the endothelial layer is a hallmark of atherosclerotic plaque development and is mediated by heavily N-glycosylated surface adhesion molecules, such as intercellular adhesion molecule-1 (ICAM-1). N-glycosylation is a key co- and post-translational modification that adds sugar molecules to Asparagine residues of surface and secreted proteins. While it has been suggested that surface and secreted proteins will not be expressed unless fully processed to a complex N-glycoform, emerging data has suggested that multiple N-glycoforms can exist on the cell surface. Previous data from our lab has shown that endothelial inflammation produces multiple N-glycoforms of ICAM-1, and that a hypoglycosylated, or high-mannose (HM), form of ICAM-1 enhances adhesion of pro-inflammatory monocytes associated with more severe atherosclerosis and adverse cardiac events. Despite these findings, little is understood about the regulation of N-glycans during disease. In this study, we focus on the α-mannosidases; an understudied class of enzymes for early N-glycan processing. We show that α-mannosidase activity decreases with TNFα treatment in endothelial cells, and this decrease correlates with HM N-glycan formation on the cell surface. Further, we demonstrate that this inhibition is class-I dependent, and is independent of NF-κB upregulation of ICAM-1. Finally, we show that this inhibition is due in part to hydrogen peroxide (H2O2), generated by Endoplasmic Reticulum oxidoreductase 1-α (ERO1α). These data provide insights into the regulation of surface N-glycans during inflammation and demonstrate a novel role for reactive species in N-glycan biosynthesis.