AbstractWith disease-modifying treatment for Parkinson’s disease (PD) associated with variants in the glucocerebrosidase gene (GBA1) under way, the challenge to design clinical trials with non-PD-manifest GBA mutation carriers (GBA1_NMC) comes within close reach. To delineate trajectories of motor and non-motor markers as well as serum neurofilament light (sNfL) levels and to evaluate clinical endpoints as outcomes for clinical trials in GBA1_NMC, longitudinal data of 56 GBA1_NMC carriers and 112 age- and sex-matched GBA1 wildtype participants (GBA1_wildtype) with up to 9 years of follow-up was analyzed using linear mixed-effects models (LMEM) and Kaplan–Meier survival analysis of clinical endpoints for motor and cognitive function. GBA1_NMC showed worse performance in Pegboard, 20 m fast walking, global cognition as well as in executive and memory function at baseline. Longitudinally, LMEM revealed a higher annual increase of the MDS-UPDRS III bradykinesia subscore in GBA1_NMC compared to GBA1_wildtype, but comparable trajectories of all other motor and non-motor markers as well as sNfL. Kaplan–Meier survival analysis showed a significantly earlier progression to clinical endpoints of cognitive decline in GBA1_NMC. Incidence of PD was significantly higher in GBA1_NMC. In conclusion, our study extends data on GBA1_NMC indicating early cognitive decline as a potentially characteristic feature. Comprehensive longitudinal assessments of cognitive function are crucial to delineate the evolution of early changes in GBA1_NMC enabling a more accurate stratification and allow for a more precise definition of trial design and sample size.