The RASopathies are genetic syndromes associated with pathogenic variants causing dysregulation of the Ras/mitogen-activated protein kinase (Ras-MAPK) pathway, essential for brain development, and increased risk for neurodevelopmental disorders. Yet, the effects of most pathogenic variants on the human brain are unknown. We examined: (1) How Ras-MAPK activating variants ofPTPN11/SOS1protein-coding genes affect brain anatomy. (2) The relationship betweenPTPN11gene expression levels and brain anatomy, and (3) The relevance of subcortical anatomy to attention and memory skills affected in the RASopathies. We collected structural brain MRI and cognitive-behavioral data from 40 pre-pubertal children with Noonan syndrome (NS), caused byPTPN11(n= 30) orSOS1(n= 10) variants (age 8.53 ± 2.15, 25 females), and compared them to 40 age- and sex-matched typically developing controls (9.24 ± 1.62, 27 females). We identified widespread effects of NS on cortical and subcortical volumes and on determinants of cortical gray matter volume, surface area (SA), and cortical thickness (CT). In NS, we observed smaller volumes of bilateral striatum, precentral gyri, and primary visual area (d’s < −0.8), and extensive effects on SA (d’s > |0.8|) and CT (d’s > |0.5|) relative to controls. Further, SA effects were associated with increasingPTPN11gene expression, most prominently in the temporal lobe. Lastly,PTPN11variants disrupted normative relationships between the striatum and inhibition functioning. We provide evidence for the effects of Ras-MAPK pathogenic variants on striatal and cortical anatomy as well as links betweenPTPN11gene expression and cortical SA increases, and striatal volume and inhibition skills. These findings provide essential translational information on the Ras-MAPK pathway’s effect on human brain development and function.