Protein monoaminylation is a biochemical process through which biogenic monoamines (e.g., serotonin, dopamine, histamine, etc.) are covalently bonded to certain protein substrates via Transglutaminase 2, an enzyme that catalyzes the transamidation of primary amines to the γ-carboxamides of glutamine residues. Since their initial discovery, these unusual post-translational modifications have been implicated in a wide variety of biological processes, ranging from protein coagulation to platelet activation and G-protein signaling. More recently, histone proteins – specifically histone H3 at glutamine 5 (H3Q5) – have been added to the growing list of monoaminyl substrates in vivo, with H3Q5 monoaminylation demonstrated to regulate permissive gene expression in cells. Such phenomena have further been shown to contribute critically to various aspects of (mal)adaptive neuronal plasticity and behavior. In this short review, we examine the evolution of our understanding of protein monoaminylation events, highlighting recent advances in the elucidation of their roles as important chromatin regulators.