Fast-paced lifestyles result in consumers replacing traditional meals with on-the-go snack foods. In general, snacks are higher in saturated fats and simple sugars, and pose health concerns for consumers, which prompts the need for healthy nutritious alternatives to common snacks. Proso millet is a nutritious, and fast growing gluten free cereal. Spent grain (SG), the main by-product of brewing and distilling, contains high amount of protein and insoluble fibers. This study utilized proso millet and spent grain in the production of an extruded expanded snack, demonstrating their appropriateness as an ingredient in food production. The first objective of this study was to determine the effect of extrusion processing conditions (moisture, barrel temperature, screw speed) on the physical, chemical, and functional properties of proso millet based extruded snack. The second objective of this research was to determine the effect of distiller’s spent grain (DSG) addition level and particle size on physicochemical and functional properties of extruded snacks. The third objective of this research was to evaluate the effect of spent grain type and particle size on the physicochemical and functional properties of extruded snacks. Samples were produced using a co-rotating twin-screw 25:1 L/D laboratory scale extruder.