Background <p>Endovascular symptoms are among the most debilitating long COVID symptoms; however, underlying mechanisms are unclear. Children and young adults with long COVID, an understudied population, offer key insight into long COVID pathology.</p> Methods <p>Eighty-four children and young adults ≤25 years from the U.S. and Canada were enrolled; 61 with long COVID and 23 healthy pediatric controls. We assessed symptom burden, quantified fibrin amyloid microclots, endovascular cytokines, cell-free DNA, and conducted in vitro assays to assess Spike-related neutrophil-mediated endothelial cell injury.</p> Results <p>Cardiovascular symptoms were prevalent among participants with long COVID. Microclot burden was increased (<i>p</i> = 0.0003), as were markers of angiogenesis and endothelial remodeling, including FGF-2, which correlated with microclots (<i>p</i> = 0.04). Cytokines involved in leukocyte trafficking (sVCAM-1, L-selectin, α-2-macroglobulin) were reduced while cell-free DNA, a marker of intravascular neutrophil extracellular trap (NET) formation, was increased (<i>p</i> = 0.003) and positively correlated with microclot component serum amyloid A (<i>p</i> = 0.004). Co-culture assays revealed that NETosis, triggered by Spike immune complexes, contributes to endothelial injury in long COVID.</p> Conclusions <p>Children and young adults with long COVID with cardiovascular symptoms display increased microclots, endothelial injury, and neutrophil inflammation, which warrant further evaluation and suggest intravascular NETosis as a key driver of endovascular pathology in long COVID.</p> Impact <p><UnorderedList Mark="Bullet"> <ItemContent> <p>Children and young adults with long COVID display elevated endothelial biomarkers, underscoring disease-related rather than age-related endovascular profiles following SARS-CoV-2 infection.</p> </ItemContent> <ItemContent> <p>Children and young adults with long COVID exhibit increased microclot burden in blood.</p> </ItemContent> <ItemContent> <p>Neutrophil activation may contribute to ongoing endovascular injury in long COVID.</p> </ItemContent> <ItemContent> <p>A combination of microclots, neutrophil markers, and endothelial cytokines could serve as biomarkers for Long COVID.</p> </ItemContent> </UnorderedList></p>