Little is currently known about the mechanisms by which pathogenic variants of FGFR2 produce changes in the FGFR protein and influence the clinical presentation of affected individuals. We report on a patient with a de novo pathogenic variant of FGFR2 and a phenotype consistent with Jackson-Weiss syndrome who presented with delayed, rapidly progressive multisutural craniosynostosis and associated medical complications. Using 3-dimensional modeling of the FGFR protein, we provide evidence that this variant resulted in abnormal dimerization and constitutive activation of FGFR, leading to the Jackson-Weiss phenotype. Knowledge regarding the correlation between genotype and phenotype of persons with FGFR2-related craniosynostosis has the potential to allow for anticipation of medical complications, institution of early treatment, and improved clinical outcomes.