Metallic magnesium is mostly used in lightweight alloys, one of the main purposes of which being to decrease fuel consumption in automotive applications. Several different production processes are currently used worldwide, and the magnesium production scenario is changing rapidly, with Chinese production increasingly dominating the world market. For the purpose of this paper, a novel approach to Life Cycle Assessment (LCA) was applied, in which Material Flow Accounting (MFA) and energy-based methods (Embodied Energy, Exergy Analysis and Emergy Synthesis) are integrated and complemented by a detailed description of the environmental hazards associated with emissions from all the different processes. Furthermore, the findings of the assessment are applied to estimate the Global Warming Potential (GWP) and the Acidification Potential (AP) related to the world primary magnesium production in 2007, with an allocation to the producer countries. Results highlight the relevance in terms of environmental impact of the country where the Mg production process takes place, and clearly identify the dominating Chinese Pidgeon process as the least sustainable Mg production chain, despite its growing diffusion: it has the highest environmental burdens as well as material and energy consumption, and the lowest exergy efficiency.