2024/MB8 dissimilar alloys can be joined in atomsphere using ultrasonic-assisted transient liquid phase (U-TLP) bonding with multi-interlayers made of copper and zinc foil. These multi-interlayers not only prevent the formation of Al3Mg2 and Al12Mg17 but also offer high efficiency and defect-free joining at 380 °C. The results show that the copper foil functions as a “physical barrier”. The zinc foil lowers the bonding temperature and balances the consumption of copper foil via base metals (BMs). Mechanical properties of these joints can be improved by adjusting the type of intermetallic compounds (IMCs) on the surface of copper foil. When utilizing Cu interlayer, the fracture route of the joints occurred on the Al2Cu layer between Al BM and copper foil. When utilizing Cu/Zn multi-interlayer, the fracture route shifted to the Mg2Cu layer along the surface of copper foil on the Mg BM side as the IMCs layer between Al BM and copper foil changed from Al2Cu to AlZnCu2 +Al2Cu, improving bonding. When utilizing Zn/Cu/Zn multi-interlayer, the IMCs layer along the surface of copper foil on the Mg BM side changed from MgZn2 (1.5 s) to Mg4Zn7 + MgZnCu (3 s) to MgZnCu (11 s) to Mg2Cu (24 s) with longer ultrasonic treatment time. The MgZnCu layer increased the shear strength of the joints to a maximum value of 59.92 MPa, which is about threefold that of the Cu interlayer and was about twofold that of the Cu/Zn multi-interlayer.