Green waste (GW) represents 11% of municipal solid waste. Managing this waste is challenging due to its physicochemical variability, low density, and presence of hard-to-degrade lignocellulosic compounds. Composting is an alternative for GW transformation and valorization. However, due to the substrate characteristics, processing times are long, and the end product typically does not meet quality standards. Incorporating additives and co-substrates are operational strategies that contribute to overcoming these challenges. An essential step is the determination of a mixture’s composition that ensures synergistic effects on the process and end-product quality. This research assessed the effect of adding biochar (Bch) in the co-composting of GW and food waste (FW). A previously studied co-composting mixture (M) of GW, raw and processed FW, sawdust (Sd), and phosphoric rock (Pr) with four treatments by duplicate were assessed at the pilot scale: T1: 100% GW, T2: M1, T3: M2 + 2% Bch, y T4: M3 + 5% Bch. The results show that Bch treatments maintained the range of thermophilic temperatures for longer than the other two treatments (between four and five additional days), showing greater biological activity and better end-product hygienization. Likewise, in the Bch treatments, the hemicellulose and cellulose degradation improved compared to treatments without Bch by 33.9% and 23.3%, respectively, and nitrogen losses were reduced by up to 70%. Regarding the end product, adding a 2% dose of Bch allowed the highest fertility index compared to the other three treatments, showing its potential for agricultural use. This work demonstrates that adding biochar to FW and GW co-composting improves organic matter degradation rates, lignocellulosic degradation, and end-product quality.