Cancer is an extremely complex disease characterized by abnormal cell growth due to genetic and environmental factors. With the rise of the field of epigenetic transcriptomics, 5-methylcytidine (mC) modification has been identified as one of the most common chemical modifications occurring in various RNA types. The writers, erasers, and readers of mC modification regulate cancer initiation, progression, and therapeutic responses, such as the proliferation, metastasis, angiogenesis, metabolic reprogramming, immune escape, and therapeutic resistance of tumour cells, by regulating RNA stability, translation, nuclear export, and splicing processes. In this review, we elucidate the biological process of mC modification, summarize the abnormal expression of RNA-modifying proteins (RMPs) in common malignant tumours, explore their functional effects on malignant hallmarks of cancer and molecular mechanisms, and prospect the potential clinical application value of mC.