Beyond storing and supplying energy in the liver and muscles, glycogen also plays critical roles in cell differentiation, signaling, redox regulation, and stemness under various physiological and pathophysiological conditions. Such versatile functions have been revealed by various forms of glycogen storage diseases. Here, we outline the source of carbon flux in glycogen metabolism and discuss how glycogen metabolism guides CD8⁺ T‐cell memory formation and maintenance. Likewise, we review how this affects macrophage polarization and inflammatory responses. Furthermore, we dissect how glycogen metabolism supports tumor development by promoting tumor‐repopulating cell growth in hypoxic tumor microenvironments. This review highlights the essential role of the gluconeogenesis‐glycogenesis‐glycogenolysis‐PPP metabolic chain in redox homeostasis, thus providing insights into potential therapeutic strategies against major chronic diseases including cancer.