Adenomatous Polyposis Coli (APC) is a tumor suppressor and essential negative regulator of the Wnt signaling pathway. Wnt signaling is crucial for proper patterning and cell fate specification during development and regulates stem cell homeostasis throughout adulthood. Mutations in Apc are strongly linked to human colorectal cancers and these mutations aberrantly activate Wnt signaling. How APC regulates the Wnt pathway and how oncogenic Apc mutations activate Wnt signaling and promote tumorigenesis are not fully understood. To address these questions, we utilized in vitro reconstitution assays, as well as Apc knockdown or mutation in human cells, zebrafish, and mice. We find APC directly enhances activity of Glycogen Synthase Kinase-3 (GSK-3), an essential regulator of diverse signaling pathways. Furthermore, APC loss of function mimics GSK-3 inhibition by reducing phosphorylation of Glycogen Synthase and by activating mechanistic Target of Rapamycin in Complex 1 (mTORC1). Thus we identify novel roles for APC as a regulator of GSK-3 activity and GSK-3 dependent signaling. Wnts reduce GSK-3 activity through an unknown mechanism to activate downstream signaling. We find Wnts induce rapid APC dissociation from the GSK-3/Axin complex, suggesting a new mechanism for how Wnts reduce GSK-3 activity. GSK-3 also regulates mTORC1, and we find oncogenic Apc mutations activate mTORC1. Furthermore, many Apc mutant phenotypes are partially rescued by mTORC1 inhibition, while others are rescued by combined mTORC1 and Wnt inhibition. These observations demonstrate roles for mTORC1 activation downstream of Apc mutation. Taken together, our findings provide new insight into APC function and the effects of oncogenic Apc mutation.