Lightweight Mg alloys are appealing as structural materials for improving energy efficiency in various applications. However, the tradeoff between strength and deformability of Mg at room temperature is a major obstacle to widespread use of Mg alloys. The plasticity of Mg is particularly related to 〈c + a〉 dislocations, which dominates c-axis strain, while it is rarely activated under ambient conditions because of a high critical resolved shear stress. Here, we prepare a lamellar-structured Mg/Mg₂Sn alloy consisting of two phases, Mg and Mg₂Sn, arranged alternately with high-density phase interfaces, which shows a synergy of high strength and deformability. High-density Mg/Mg₂Sn interfaces act as strong barriers for dislocations motion, obstructions for crack propagation, and efficient 〈c + a〉 dislocation sources, which contribute to the strengthening and plasticity of the laminated Mg/Mg₂Sn alloy. This unique interface-mediated plasticity provides a new pathway to improve the mechanical properties of hexagonal close-packed Mg alloys.