The polarity and adhesion of radial glial cells (RGCs), which function as progenitors and migrational guides for neurons, are critical for morphogenesis of the cerebral cortex. These characteristics largely depend on cadherin-based adherens junctions, which anchor apical end-feet of adjacent RGCs to each other at the ventricular surface. Here, we show that mouse numb and numb-like are required for maintaining radial glial adherens junctions. Numb accumulates in the apical end-feet, where it localizes to adherens junction–associated vesicles and interacts with cadherins. Numb and Numbl inactivation in RGCs decreases proper basolateral insertion of cadherins and disrupts adherens junctions and polarity, leading to progenitor dispersion and disorganized cortical lamination. Conversely, overexpression of Numb prolongs RGC polarization, in a cadherin-dependent manner, beyond the normal neurogenic period. Thus, by regulating RGC adhesion and polarity, Numb and Numbl are required for the tissue architecture of neurogenic niches and the cerebral cortex.