Malic enzyme 1 (ME1) plays a key role in promoting malignant phenotypes in various types of cancer. ME1 promotes epithelial-mesenchymal transition (EMT) and enhances stemness via glutaminolysis, energy metabolism reprogramming from oxidative phosphorylation to glycolysis. As a result, ME1 promotes the malignant phenotypes of cancer cells and poor patient prognosis. In particular, ME1 expression is promoted in hypoxic environments associated with hypoxia-inducible factor (HIF1) α. ME1 is overexpressed in budding cells at the cancer invasive front, promoting cancer invasion and metastasis. ME1 also generates nicotinamide adenine dinucleotide (NADPH), which, together with glucose-6-phosphate dehydrogenase (G6PD) and isocitrate dehydrogenase (IDH1), expands the NADPH pool, maintaining the redox balance in cancer cells, suppressing cell death by neutralizing mitochondrial reactive oxygen species (ROS), and promoting stemness. This review summarizes the latest research insights into the mechanisms by which ME1 contributes to cancer progression. Because ME1 is involved in various aspects of cancer and promotes many of its malignant phenotypes, it is expected that ME1 will become a novel drug target in the near future.