Uterine fibroids (UFs) or leiomyoma are frequently associated with somatic mutations in the mediator complex subunit 12 (MED12) gene; however, the function of these mutations in human UF biology is yet to be determined. Herein, we determined the functional role of the most common MED12 somatic mutation in the modulation of oncogenic Wnt4/β-catenin and mammalian target of rapamycin (mTOR) signaling pathways. Using an immortalized human uterine myometrial smooth muscle cell line (UtSM), we constitutively overexpressed either MED12-Wild Type or the most common MED12 somatic mutation (c.131G>A), and the effects of this MED12 mutation were compared between these cell lines. This immortalized cell line was used as a model because it expresses wild type MED12 protein and do not possess MED12 somatic mutations. By comparing the effect between MED12-WT and MED12-mutant (mut) stable cell populations, we observed increased levels of protein expression of Wnt4 and β-catenin in MED12-mut cells as compared with MED12-WT cells. MED12-mut cells also expressed increased levels of mTOR protein and oncogenic cyclin D1 which are hallmarks of cell growth and tumorigenicity. This somatic mutation in MED12 showed an effect on cell-cycle progression by induction of S-phase cells. MED12-mut cells also showed inhibition of autophagy as compared with MED12-WT cells. Together, these findings indicate that the MED12 somatic mutation has the potentials for myometrial cell transformation by dysregulating oncogenic Wnt4/β-catenin and its downstream mTOR signaling which might be associated with autophagy abrogation, cell proliferation, and tumorigenicity.