Most of the beverages have a high glycemic index, which is attributed to a sudden rise in blood sugar. The beneficial role of functional foods combination provided the tool to perform and design our study to develop an instant beverage mix (IBM) that might be revealed as the favorable therapeutic potential for the treatment of hyperglycemia and act as a functional beverage. Therefore, resistant fibre-rich ingredients/raw materials were used to formulate the cereal-based instant beverage (CIB). CIB was formulated using black rice flour (40–70%), germinated lentil flour (10–20%), sweet potato flour (10–20%), and mulberry powder (10–20%). The product formulation was optimized with respect to the following responses such as color and appearance, texture, flavor, taste, and overall acceptability using a D-optimal mixture design. The results revealed that the variation in raw ingredients significantly affected the organoleptic properties of trials. The ratio 40 : 20 : 20 : 20 of black rice: germinated lentil: sweet potato: mulberry was found to be optimum for the development of CIB. Optimized CIB had 9.71 ± 0.10 g/100 g of crude protein, 4.73 ± 0.09 g/100 g of fat, and 4.48 ± 0.06 g/100 g of crude fibre. Moreover, the total mineral content and carbohydrate content were found to be 1.08 ± 0.07 g/100 g and 72.45 ± 0.44 g/100 g, respectively, whereas, the energy value was 371.21 ± 4.23 kcal. In vivo glycemic index was also performed for the optimized CIB. The findings showed a lower glycemic response (37.70) than the diabetic control group, and blood glucose was found to be lowered (279.67 ± 20.06 to 227.17 ± 13.44 mg/dL) via the hypoglycemic mechanism. Thus, the optimized CIB exhibited a therapeutic effect against diabetic conditions and might be a healthy instant beverage for human consumption.