Biochar, a fine-grained porous material, exhibits properties that improve soil quality on agricultural land. Biochar, in combination with mineral fertilizers in perennial mixed crops, has so far not been studied for its effect on biomass production and feed value. The study, conducted in 2021 and 2022, aims to investigate the impact of different biochar application rates (alone and in combination with high and low NPK (nitrogen, phosphorus, potassium) fertilizer dosages) on the yield and structural carbohydrate content in grass–legume mixtures. Thus, a two-factorial pot study consisting of 36 pots was established in 2021. The study was set up in a randomized block design with nine fertilization treatments in four replicates. The factors studied were the rate of biochar (0, 5, and 10 Mg ha−1) and the rate of NPK mineral fertilizer application (0, 105, and 185 kg ha−1). First, biochar was applied and then the grass–legume mixture was sown, consisting of Lolium perenne L., Festuca arundinacea Schreb., Dectylis glomerata L., Lolium multiflorum Lam., Phleum pratense L., Festuca rubra L., Festuca ovina L., Poa pratensis L., Trifolium repens L., and Medicago sativa L. In both years of the study, during the growing season, plant biomass was harvested three times to assess biomass production and structural carbohydrate content. In the collected samples, neutral detergent fiber (NDF), acid detergent fiber (ADF), and acid detergent lignin (ADL) were examined, and, based on these, dry matter digestibility (DDM), dry matter intake (DMI), and relative feed value (RFV) were calculated. Compared to the control, the biochar addition increased the biomass production by 6.7–14.4% in the first year and by 49–59% in the second year after application. The addition of biochar in combination with NPK fertilization caused an increase in biomass production—22–45% in the first year and 71–136% in the second year after application. The structural carbohydrate content in the mixture depended neither on the biochar dose nor on fertilization. On the other hand, the most significant differences for the studied parameters were observed due to different harvest dates.