As a cantilever failure involves rapid channel widening and delivers a large volume of sediment into the channel, it is important to understand this mechanism from the river engineering point of view. Different types of riverbank failures have been investigated in several previous studies, but these works have limitations in understanding the complex mechanisms of cantilever failure regarding the coupling of fluvial erosion with that failure. Experiments were thus carried out to assess the underlying mechanisms of cantilever failure using three types of cohesive materials classified on the basis of their percentages of silt-clay content. The experiments showed that fluvial erosion of the lower part of cohesive riverbanks progressively undermines the upper part during the initial stage of a cantilever failure. Tension cracks then develop at the upper surface of the cohesive riverbanks and beam failure occurs thereafter. A mathematical model of cantilever failure was also developed. The model uses a triple-grid approach to simulate the behaviour of a cantilever within the framework of fluvial erosion and the cantilever's subsequent failure. The simulated results show good agreement with the experimental results in terms of spatial-averaged bank width and water level along cohesive riverbanks.