N-methylguanosine (mG) methylation, one of the most common RNA modifications in eukaryotes, has recently gained considerable attention. The biological functions of mG modification in RNAs, including tRNA, rRNA, mRNA, and miRNA, remain largely unknown in human diseases. Owing to rapid advances in high-throughput technologies, increasing evidence suggests that mG modification plays a critical role in cancer initiation and progression. As mG modification and hallmarks of cancer are inextricably linked together, targeting mG regulators may provide new possibilities for future cancer diagnoses and potential intervention targets. This review summarizes various detection methods for mG modification, recent advances in mG modification and tumor biology regarding their interplay and regulatory mechanisms. We conclude with an outlook on the future of diagnosing and treating mG-related diseases.