Background: Epigenetic modifications, such as transcription, DNA repair, and replication significantly influence tumour development. Aberrant gene expression and modifications can have a crucial impact on the initiation and progression of tumours. The minichromosome maintenance (MCM) protein family, which is responsible for DNA synthesis, plays a crucial role in tumorigenesis and chemotherapy resistance by regulating the cell cycle and DNA replication stress. Recent studies have shown that dysregulation of the MCMs can lead to these negative outcomes. This study aimed to examine the role of the MCM proteins in DNA synthesis in 33 types of cancers. Methods: Various public databases were used to examine the expression, methylation regulation, mutations, and functions of eight MCM proteins (MCM2–9) in pan-cancer. The study investigated the correlation between abnormal MCM expression and clinical outcomes, including prognosis and drug response. The microRNA–mRNA network upstream of the MCM genes and the downstream signalling pathways were extensively investigated to determine the molecular mechanisms that drive tumour development. Results: The study found that the MCM gene expressions differed depending on the type of cancer; high MCM gene expression was linked to poor overall survival in most cancers. Additionally, MCM gene expression was associated with various immunological features and drug sensitivity. These findings offer important insights for the development of targeted cancer therapies. Conclusions: Altogether, this study reveals that the MCM genes are differentially expressed across various cancers and are associated with clinical prognoses. These genes may influence the occurrence and development of tumours through several pathways, including the PI3K–AKT, PAS/MAPK and TSC/mTOR signalling pathways and immune-related pathways.