Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the cause of the COVID-19 pandemic, has become a global health emergency. The damage and threat posed by this virus to nearly every country in the world have far exceeded those of the other six previous coronaviruses, including Middle East respiratory syndrome coronavirus (MERS-CoV) and severe acute respiratory syndrome (SARS-CoV) combined. A comprehensive profile of the hematological and inflammatory biomarkers of COVID-19 is covered in this review. Significant hematological changes that have been reported in infected patients include the following biomarker candidates: lymphocyte counts (LYM), neutrophil counts (NØ), neutrophil to lymphocyte ratio (NLR), neutrophil to CD8+ T-cell ratio (N8R), eosinophil counts (EØ), platelet counts (PLT), and platelet to lymphocyte ratio (PLR). Likewise, significant changes in soluble mediators include interleukin (IL)-2r, IL-2r to lymphocyte ratio (ILR), IL-6, interferon-γ-induced protein 10 (IP-10), monocyte chemoattractant protein (MCP-3), and macrophage colony-stimulating factor (M-CSF). Multiplex biomarker analyses based on hematological and cytokine changes in combination with biomedical levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST), lactate dehydrogenase (LDH), N-terminal prohormone BNP (NT-proBNP), serum urea, C-reactive protein (CRP), and D-dimer have shown improved diagnostic accuracy for determining disease severity in patients compared with single biomarker analyses. Here, we provide a current review of blood/serum biomarker abnormalities associated with different levels of COVID-19 severity.