Exosomes, which are extracellular vesicles with a diameter ranging from 40 to 160 nm, are abundantly present in various body fluids. Exosomal microRNA (ex-miR), due to its exceptional sensitivity and specificity, has garnered significant attention. Notably, ex-miR is consistently detected in almost all bodily fluids, highlighting its potential as a reliable biomarker. This attribute of ex-miR has piqued considerable interest in its application as a diagnostic tool for the early detection, continuous monitoring, and prognosis evaluation of cancer. Given the critical role of exosomes and their cargo in cancer biology, this review explores the intricate processes of exosome biogenesis and uptake, their multifaceted roles in cancer development and progression, and the potential of ex-miRs as biomarkers for tumor diagnosis and prognosis.