Caragana korshinskii is an underutilized non-grain feed resource. This study investigated the optimal dietary inclusion level of fermented Caragana korshinskii for Mongolian lambs by assessing growth performance, antioxidant status, meat quality, fatty acid and amino acid profiles, and rumen microbiota. Thirty-six lambs were fed diets containing 0%, 10%, 15%, or 20% fermented Caragana korshinskii replacing Leymus chinensis hay for 105 days after a 10 d adaptation period. The 10% group exhibited higher final body weight, average daily gain, feed intake, and carcass weight (p < 0.05). All supplemented groups showed elevated serum T-AOC and reduced MDA (p < 0.05); furthermore, 10% supplementation increased CAT, and 10% and 15% supplementation increased SOD (p < 0.05). Meat quality improved in the 10% and 15% groups, with lower water loss, shear force, drip loss, L*, and b* (p < 0.05; p < 0.01), and higher a* (p < 0.05). Muscle nutrients remained unchanged. Amino acid alterations included increased Glu (all groups), Thr, Ile (15%, 20%), Leu (20%), and variable changes in Asp. Fatty acid analysis revealed higher C18:0, PUFA, C18:2n6c, C18:3n3 in 10% (p < 0.05; p < 0.01), but lower PUFA and C18:3n3 in the 15% and 20% groups (p < 0.05). Rumen 16S sequencing revealed higher abundances of Thermodesulfobacteriota, Methanobacteriota, Methanobrevibacter (10%, 20%), Xylanibacter, Succiniclasticum (15%, 20%), and lower Rikenellaceae_RC9_gut_group (20%) (p < 0.05; p < 0.01). Correlations linked rumen microbes to meat-related fatty acids and amino acids. In conclusion, supplementation with 10% fermented Caragana korshinskii optimally enhanced lamb growth, health, and meat quality through modulation of the rumen microbiota.