Our understanding of how developmental cell death contributes to the formation of brain circuits remains limited. Cajal-Retzius cells (CRs) constitute a major transient neuronal population in the mammalian neocortex, which largely disappears at the time of postnatal somatosensory maturation. In this study, we used mouse genetics, anatomical, functional and behavioral approaches to explore the impact of the early postnatal death of CRs in the maturation of the cortical circuit. We find that CRs apoptosis plays a crucial role in facilitating the proper development of inhibitory connectivity from layer 1 interneurons onto layer 2/3 pyramidal cells. Disrupting the natural apoptosis of CRs results in over-excitability of layer 2/3 both within and across cortical columns in vivo. This heightened excitability is associated with delayed learning and diminished performance in a multi-whisker-dependent texture discrimination task. CRs apoptosis is a key factor for the maturation of proper inhibition onto layer 2/3 and, consequently, for effective somatosensory processing. This novel approach of the CRs transient role in connectivity brings to light the importance of transient populations in the typical and atypical brain formation.