Agricultural byproducts such as wheat straw are abundant, low-cost feedstocks suitable for bioconversion into value-added chemicals. This study focused on developing a single-stage solid-state fermentation (SSF) process for the co-production of cellulolytic enzymes and lactic acid. A synergistic co-culture of Aspergillus niger and Lactobacillus casei was employed for the bioconversion of mild alkali-pretreated wheat straw. Key process parameters, including temperature, moisture content, and initial pH, were systematically optimized to enhance product yields. Enzyme activity was determined using spectrophotometric assays, while lactic acid concentrations were quantified using HPLC. Under optimized conditions, the process achieved the concurrent production of cellulase at a high activity rate of 75 U/g of the dry substrate and a final lactic acid titer of 98 g/L. The efficient enzymatic hydrolysis of the substrate by A. niger supplied the necessary fermentable sugars for L. casei, resulting in a sugar-to-lactic acid conversion efficiency of over 85%. These findings demonstrate that an integrated co-culture SSF is a highly effective strategy for the valorization of lignocellulosic waste. This process presents a viable and robust model for producing multiple biochemicals from a single feedstock, offering a practical contribution to the circular bioeconomy.