Colony-stimulating factor 3 (CSF3), additionally called granulocyte colony-stimulating factor (G-CSF), is the major cytokine regulating neutrophil production and also impacting their function. The actions of this cytokine are mediated through its unique receptor, the colony-stimulating factor 3 receptor (CSF3R). Several classes of pathogenic mutations in the CSF3R gene have been identified that have distinct biological properties and clinical impacts. This review provides an overview of CSF3R, the various pathogenic CSF3R mutations/variants and their biological effects. It also details the diseases to which they contribute, notably including chronic neutrophilic leukemia (CNL) and other myeloproliferative neoplasms (MPNs), myelodysplastic neoplasms (MDS), combined MDS/MPN disorders such as atypical chronic myeloid leukemia (aCML) and chronic myelomonocytic leukemia (CMML), as well as acute myeloid leukemia (AML) and lymphoid malignancies.