Abstract INTRODUCTION Sex differences are apparent in neurodegenerative diseases but have not been comprehensively characterized in frontotemporal dementia (FTD). METHODS Participants included 337 adults with autosomal dominant FTD enrolled in the ALLFTD Consortium. Clinical assessments and plasma were collected annually for up to 6 years. Linear mixed‐effects models investigated how sex and disease stage are associated with longitudinal trajectories of cognition, function, and neurofilament light chain (NfL). RESULTS While sex differences were not apparent at asymptomatic stages, females showed more rapid declines across all outcomes in symptomatic stages compared to males. In asymptomatic participants, the association between baseline NfL and clinical trajectories was weaker in females versus males, a difference that was not present in symptomatic participants. DISCUSSION In genetic FTD, females show cognitive resilience in early disease stages followed by steeper clinical declines later in the disease. Baseline NfL may be a less sensitive prognostic tool for clinical progression in females with FTD‐causing mutations. Highlights Females with genetic FTD exhibit overall steeper increases in plasma neurofilament light chain (NfL) than males. Females with genetic FTD outperform NfL levels in asymptomatic stages compared to males. Once symptomatic, females with genetic FTD decline more rapidly than males. Plasma NfL is a stronger prognostic marker in asymptomatic males than females.