Mexico is an important iron and steel manufacturer; it is the 13th largest steel producer in the world. The Mexican iron and steel industry is first in energy consumption for industrial energy use, representing 14.3% of the total industrial final energy consumption and a similar share of related carbon dioxide emissions. The aim and novelty of this paper is to estimate both the energy intensity and CO2 intensity of the Mexican iron and steel industry in 2010 based on defined system boundaries and an international comparison methodology and to compare the energy intensity of Mexican with those the US and China based on a literature review. The boundaries consider energy consumption for all coke making, pelletizing, sintering, iron making, steel making, steel casting, hot rolling, cold rolling, and processing, such as galvanizing or coating. They also include energy use for net imported pig iron, direct-reduced iron, pellets, lime, oxygen, ingots, blooms, billets, and slabs. Under these boundary conditions, the Mexican iron and steel industry was shown to be more energy efficient and less carbon intense than those the U.S. and China. The reasons for this efficiency are mainly the large shares of the electric arc furnace route (69.4%) and continuous casting (100%) in production and the large share of natural gas in the fuel mix. This paper highlights the importance of the definition of boundaries and clear methodologies to analyse the iron and steel energy efficiency.