Forty years ago, high mobility group box 1 (HMGB1) was discovered in calf thymus and named according to its electrophoretic mobility in polyacrylamide gels. Now, we know that HMGB1 performs dual functions. Inside the cell, HMGB1 is a highly conserved chromosomal protein acting as a DNA chaperone. Outside of the cell, HMGB1 is a prototypical damage-associated molecular pattern, acting with cytokines, chemokines, and growth factors. During tumor development and in cancer therapy, HMGB1 has been reported to play paradoxical roles in promoting both cell survival and death by regulating multiple signaling pathways, including inflammation, immunity, genome stability, proliferation, metastasis, metabolism, apoptosis, and autophagy. Here, we review the current knowledge of both HMGB1's oncogenic and tumor-suppressive roles and the potential strategies that target HMGB1 for the prevention and treatment of cancer.