We examined plasma GFAP moderation of AD biomarkers (amyloid beta [Aβ]‐positron emission tomography [PET][A]; plasma phosphorylated tau‐181 [p‐tau181][T1]), neurodegeneration (plasma NfL[Nplasma]; structural magnetic resonance imaging [MRI][NMRI]), and cognition (Cogmemory; Cogexecutive) in two cohorts: University of California San Francisco (UCSF) (N = 212, 91.0% non‐Hispanic/Latino White [NHLW], age = 74.7 [7.6] years, 75.9% cognitively unimpaired [CU]) and 1Florida Alzheimer's Disease Research Centers (1FLADRC; N = 582, 32.8% NHLW, age = 70.7 [8.5] years, 28.9% CU). The gene discussed is NEFL; the disease is early-onset autosomal dominant Alzheimer disease.