The growing demand for lightweight materials in the automotive and aerospace industries has driven research on joining dissimilar lightweight alloys, particularly Al and Mg alloys (Al/Mg). Friction stir welding (FSW) is a promising technique for joining Al/Mg alloys, as it works below the base metal's melting temperature, leading to refined microstructures, reduced porosity, and enhanced productivity. The strength of Al/Mg friction stir weldment depends on the evolved interface, which is primarily characterized by micromechanical interlocks, type, and intermetallic compounds (IMCs) distribution. Different interfaces for butt joints have been discussed in the literature. However, the mechanism of interfacial interaction together with the ways to enhance the interface have not been reviewed yet. This review article fills the gap by analyzing the retrospective data for process parameters and mechanical properties. Joining mechanisms and the evolution of different interfaces at the microstructural level have been discussed. Lastly, ways to enhance the interface for improved mechanical properties are explained. By offering essential insights into FSW techniques and Al/Mg weld interfaces, this review article paves the way for developing FSW procedures for Al/Mg butt welds aiming for enhanced strength and performance. This review article is expected to be of interest to researchers and engineers working in the field of FSW, particularly for Al/Mg lightweight applications. It provides an overview of the current state of knowledge and identifies key areas for future research.