Summary Mucilage, a polysaccharide‐rich substance secreted by root cap and border cells, is a key mediator of plant–soil interactions. This review traces the evolutionary origins of root mucilage from ancestral secretion mechanisms in streptophyte algae to its diverse roles in modern vascular plants. We highlight how mucilage biosynthesis relies on conserved gene families involved in polysaccharide assembly, whose phylogenomic distribution suggests that components of this machinery were already present in algal ancestors. Combining genetic, functional, and ecological evidence, we infer a stepwise evolutionary trajectory in which mucilage initially facilitated hydration and anchorage, later supporting soil structuring, plant–microbe interactions, nutrient availability, root water uptake, and drought tolerance. Fossil and comparative evidence indicates that both the secretion of mucilage and its regulatory mechanisms are deeply conserved traits. By connecting ancestral molecular and physiological functions to modern ecological roles, we demonstrate that mucilage has been a critical adaptation enabling plants to colonize and thrive in terrestrial environments over geological timescales.