Understanding the inconsistency in the effects of monsoon changes on drought and flood occurrences would allow scientists to identify useful indicators in the prediction and early warning of regional drought and flood. Based on the calculation of the Standardized Precipitation Index, monsoon indices, and water vapor fluxes from 1956 to 2015, the relationships between drought–flood occurrences and monsoons in different regions of the Lancang River Basin were investigated. Drought and flood occurrences had spatial differences. Areas located in the lower basin had high drought and flood occurrences. The frequencies of drought and flood occurrences have no obvious regional difference and mainly varied periodically at 3–5, 8–15, and 20–25 years. Because the impact and strength of the Tibetan Plateau Monsoon (TPM) and South Asian monsoon (SAM) are limited, the TPM and SAM are the key factors that affect the occurrences of drought and flood in the upstream and downstream regions of the Lancang River Basin, respectively. The TPM and SAM are potentially useful indicators in the prediction of drought and flood occurrences. These results are of great scientific merits in developing an effective mitigation strategy to reduce the impacts of drought–flood disasters in the Lancang River Basin.