Efforts to assess the environmental performances of beef production systems often culminate in mitigation strategies without factoring in farm economics. The objective of this study was to co-assess the environmental impacts and economic performances of French suckler-beef production systems based on commercial farm data. We coordinated a technical–economic survey on 59 Charolais suckler-cattle farms in order to calculate GHG emissions and non-renewable energy (NRE) consumption over the years 2010 and 2011. Using real-world data from a farm network instead of modeled or experimental data enabled us to analyze the variability of the results and its determinants. The main variables impacting GHG emissions and NRE consumption per kg of beef (live weight) produced are (i) animal productivity (kg of live weight produced per LU), (ii) farm size (area and herd), and (iii) degree of specialization in beef production (share of cattle revenue in total farm revenue). The large, diversified farms (mixed crop–livestock farming systems) have a more negative environmental impact than the moderate-sized, specialized (beef production) farms. Animal productivity performances decrease with increasing herd size, and inputs use is below-optimal in the most strongly diversified farms. A comparison of the group of farms with the lowest and highest GHG emissions per kg beef (50% difference on GHG emissions per kg of live weight) confirmed these correlations. Through better animal productivity performances and lower use of inputs, the less-GHG-emitting farms also generate higher income per worker (+30%) while consuming less NRE. Our findings argue against the idea that size and diversification bring economic and environmental economies of scale and scope in suckler-beef production systems.