This is the first study investigating the effects of lignosulfonate (LS) properties and reaction conditions on the precipitation of proteins from green biomass juice for producing monogastric animal feed. Both the improvement in the protein precipitation yield and the resulting impact on the quality of the protein pellets were investigated with new methods and approaches. High molecular weight (MW) LS gave the highest precipitation yield, 21–33% higher than traditional heat and isoelectric methods. Electrostatic interaction between LS and soluble proteins was observed to be higher in high MW LS than others, and likely improved the precipitation. A response surface design experiment showed that pH was more important than LS concentration for precipitation. The optimum conditions predicted by the model improved precipitation by up to 61% compared to traditional methods. Using high MW LS at near optimum conditions resulted in more LS being retained in the pellet, with a final LS content of up to 60 g/kg dry matter (DM). The improved precipitation yield came at the expense of lower crude protein (CP) content of the pellet. Despite reduced pellet protein purity, the total CP recovery from the juice was improved by up to 800 g/kg DM. The in vitro protein digestibility (IVPD) and amino acid composition were not significantly affected by the presence of LS. Using high MW LS at optimum reaction conditions can significantly improve protein precipitation yield from green biomass juice without compromising digestibility. However, finding the compromise between yield and protein purity is necessary to improve feasibility.