The E3 ubiquitin ligase Ube3a is biallelically expressed in mitotic cells, including neural progenitors and glial cells, raising the possibility that UBE3A gain-of-function mutations might cause neurodevelopmental disorders irrespective of parent-of-origin. To test this possibility, we engineered a mouse line that harbors an autism-linked UBE3A-T485A (T508A in mouse) gain-of-function mutation and evaluated phenotypes in animals that inherited the mutant allele paternally, maternally, or from both parents. We found that both paternally and maternally expressed UBE3A-T485A resulted in elevated UBE3A activity in neural progenitors and glial cells where Ube3a is biallelically expressed. Expression of UBE3A-T485A from the maternal allele, but not the paternal one, led to a persistent elevation of UBE3A activity in postmitotic neurons. Maternal, paternal, or biparental inheritance of the mutant allele promoted embryonic expansion of Zcchc12 lineage interneurons which mature into Sst and Calb2 expressing interneurons, and caused a spectrum of behavioral phenotypes that differed by parent-of-origin. Phenotypes were distinct from those observed in Angelman syndrome model mice that harbor a Ube3a maternal loss-of-function allele. Our study shows that the UBE3A-T485A gain-of-function mutation causes distinct neurodevelopmental phenotypes when inherited maternally or paternally. These findings have clinical implications for a growing number of disease-linked UBE3A gain-of-function mutations. Single-cell RNA-seq (drop-seq) of mouse cerebral cortex from Ube3a-WT, Ube3a-T485A homozygotes, assessed at two ages: E14.5 and P0