Progression of primary cancer to metastatic disease is the most common cause of death in cancer patients with minimal treatment options available. Canonical drugs mainly target the proliferative capacity of cancer cells, which often leaves slow-proliferating, persistent cancer cells unaffected. Thus, we aimed to identify metabolic determinants that enable cell plasticity and foster treatment resistance and tumor escape. Using a panel of anti-cancer drugs, we uncovered that antifolates, despite inducing strong growth arrest, did not impact the cancer cell's motility potential, indicating that nucleotide synthesis is dispensable for cell motility. Prolonged treatment even selected for more motile cancer subpopulations. We found that cytosolic inhibition of DHFR by MTX only abrogates cytosolic folate cycle, while mitochondrial one-carbon cycle remains highly active. Despite a decreased cellular demand for biomass production, de novo serine synthesis and formate overflow are increased, suggesting that mitochondria provide a protective environment that allows serine catabolism to support cellular motility during nucleotide synthesis inhibition. Enhanced motility of growth-arrested cells was reduced by inhibition of PHGDH-dependent de novo serine synthesis and genetic silencing of mitochondrial one-carbon cycle. In vivo targeting of mitochondrial one-carbon cycle and formate overflow strongly and significantly reduced lung metastasis formation in an orthotopic breast cancer model. In summary, we identified mitochondrial serine catabolism as a targetable, growth-independent metabolic vulnerability to limit metastatic progression.