Most models for granular flow mobility assume constant basal friction, neglecting the dynamic effects of particle segregation on basal resistance. Combining physical experiments with discrete element method (DEM) simulations, this study deciphers how particle-size contrast and ice content control flow mobility by regulating segregation in rock-ice mixtures. Under non-segregating (slump) conditions, the intrinsic mobility of the mixture increases linearly with ice fraction and is modulated by the size ratio. At the micro-scale, this is governed by the interplay between the geometric filling effect of fine particles and their intrinsic frictional properties: for ice fines, low friction acts in synergy with filling to further reduce interparticle interlocking; for rock fines, high friction competes with the filling effect, counteracting its potential lubricating role. Under dynamic, segregating (flume) flow conditions, segregation dictates mobility: when ice particles are smaller, their downward segregation forms a low-friction basal lubricating layer that dramatically reduces the effective basal friction coefficient and enhances run-out. Conversely, when rock particles are smaller, their downward segregation sustains a highfriction base, inhibiting mobility. These contrasting behaviors underscore that effective basal friction is not a material constant but an emergent, segregation-dependent property. This framework challenges constant-friction assumptions and advances the physics-based prediction of multi-component granular flows.