Combining several animal species to optimise the performance of the whole farming system is one of the core tenets of agroecology. Here, we associated sheep with beef cattle (40–60% livestock units (LU)) in a mixed system (MIXsys) and compared its performances to those of a specialised beef cattle-only system (CATsys) and a specialised sheep-only system (SHsys). All three systems were designed to have identical annual stocking rates and similar farm areas, pastures and animals. The experiment was conducted for four campaigns (2017–2020) in an upland setting exclusively on permanent grassland under certifiedorganic farming standards. The young animals were fattened almost exclusively with forages: at pasture for lambs and indoors with haylage in winter for young cattle. Abnormally dry weather conditions led to hay purchases. We compared between-system and between-enterprise performances based on technical, economic (gross product, expenses, margins, income), environmental (greenhouse gas emissions (GHG), energy consumption) and feed–food competition balance indicators. The mixed-species association only benefited the sheep enterprise, with +17.1% meat production per LU (P < 0.03), 17.8% concentrate used per LU (P < 0.02), +10.0% gross margin (P < 0.07) and +47.5% income per LU (P < 0.03) in MIXsys vs SHsys, as well as environmental performance benefits via a reduction of 10.9% in GHG emissions (P < 0.09) and 15.7% in energy consumption (P < 0.03), and a 47.2% improvement in feed–food competition (P < 0.01) in MIXsys vs SHsys. These results are due to both better animal performance and lower concentrate consumption in MIXsys, as presented in a companion paper. These benefits outweighed the additional costs of the mixed system, especially for fencing, in terms of net income per sheep LU. There were no betweensystem differences in productive and economic performance (kilos live-weight produced, kilos concentrate used and income per LU) for the beef cattle enterprise. Despite good animal performances, the beef cattle enterprises in both CATsys and MIXsys had poor economic performance due to large purchases of conserved forages and difficulty selling the animals, which were ill-adapted to the traditional downstream sector. This multiyear study at the farming-system level, which has thus far been underresearched for mixed livestock farming systems, highlighted and quantified the benefits for sheep when combined with beef cattle on economic, environmental, and feed–food competition performance.