In this study, ultrasonic-assisted soldering was applied to join a Ta alloy with the light structural material of a Mg alloy. The effects of the ultrasonication time on the microstructure and mechanical properties of the Ta/Sn/Mg joints were investigated. The results showed that molten Sn could wet the substrates of Mg and Ta without flux under the effect of ultrasonic vibration and that the maximum shear strength of the joint was approximately 27 MPa. A large amount of Mg entered into the Sn with the effect of ultrasonic vibration and was present in molten Sn in the form of both solute and primary Mg2Sn phases. During the cooling of the joint, the supersaturated Mg solute was precipitated at the area close to the Ta sheet by way of the growth of the primary Mg2Sn phase and the nucleation of Mg2Sn on the Ta surface and in the Sn matrix. Finally, a large amount of Mg2Sn formed at the substrate/solder interface and in the Sn matrix, and the Mg2Sn at the Ta/solder interface increased the interfacial strength. With an increase in the ultrasonication time, the number of Mg2Sn phases in soldering seam was increased, but the nucleated Mg2Sn at the Ta/solder interface decreased, which resulted in a gradual decrease in the strength of the Ta/solder interface.