Variants of uncertain significance pose a difficult diagnostic and therapeutic problem, even when the candidate gene has previously been associated with Mendelian disease. Variants in SLC6A1 result in a rare neurodevelopmental disorder characterized by a variable clinical presentation of symptoms including developmental delay, epilepsy, motor dysfunction, and autism spectrum disorder. SLC6A1 encodes the γ-aminobutyric acid (GABA) transporter (GAT-1) protein that is responsible for the reuptake of GABA from the synapse to maintain homeostasis of excitatory and inhibitory neurotransmitters. SLC6A1 haploinsufficiency has been confirmed as the predominant pathway of SLC6A1-related neurodevelopmental disorders, however, the molecular mechanism underlying the variable presentation of clinical features remains unclear. Here, through work of the Undiagnosed Diseases Network, we identify an undiagnosed individual with an inherited p.(A334S) variant of uncertain significance. Pedigree analysis identifies variable symptoms between family members with the same variant. To resolve this case, we first compare proband phenotypes to a cohort of cases diagnosed with SLC6A1-related neurodevelopmental disorders and find significant symptom overlap. We then functionally compare this variant to other variants by creating an allelic series in the fruit fly Drosophila melanogaster. We identify phenotypes in flies expressing SLC6A1 variants consistent with a loss-of-function mechanism and conclude that the p.(A334S) variant is a hypomorphic allele. Through clinical and functional genomics, we begin to elucidate the underlying variability in SLC6A1-neurodevelopmental disorders. These insights will inform clinical diagnosis, prognosis, treatment planning, and aid in the design of targeted therapeutics for those living with SLC6A1-related neurodevelopmental disorders.