The mammalian neocortex is populated by different neuronal and glia cell types. During development, excitatory projection neurons and certain glia lineages emerge from radial glial progenitors (RGP). Impairments in RGP progenitor behavior and lineage progression can affect neuron production and maturation, leading to cortical malformations such as micro- or macrocephaly. The Notch signaling pathway instructs stem cell self-renewal and differentiation in a variety of tissues, including the brain. The Notch1 receptor is expressed in RGPs during embryonic development and maintains the progenitor pool. Previous studies report that loss of Notch1 leads to early progenitor differentiation and exhaustion, reducing cortical thickness. However, the precise mechanisms how Notch1 ablation leads to RGP deficits at single cell level have not been resolved. Here, we address the cell-autonomous function of Notch1 at single-cell level by using Mosaic Analysis with Double Markers (MADM). In line with previous findings, sparse deletion of Notch1 results in a reduction of mutant neurons when compared to the surrounding wild-type neurons in adult neocortex. The cortical astrocyte population is even more dramatically reduced upon sparse Notch1 deletion. Altogether, our data demonstrate the exceptional ability for MADM to unravel the true cell-autonomous role of Notch1 in RGP lineage progression and open up new avenues for in-depth molecular analysis.