Folate-mediated one-carbon metabolism (FOCM) is compartmentalized within human cells to the cytosol, nucleus, and mitochondria. The recent identifications of mitochondria-specific, folate-dependent thymidylate [deoxythymidine monophosphate (dTMP)] synthesis together with discoveries indicating the critical role of mitochondrial FOCM in cancer progression have renewed interest in understanding this metabolic pathway. The goal of this narrative review is to summarize recent advances in the field of one-carbon metabolism, with an emphasis on the biological importance of mitochondrial FOCM in maintaining mitochondrial DNA integrity and mitochondrial function, as well as the reprogramming of mitochondrial FOCM in cancer. Elucidation of the roles and regulation of mitochondrial FOCM will contribute to a better understanding of the mechanisms underlying folate-associated pathologies.