Milk protein concentrate (MPC) is a high-protein, low-lactose dairy powder that contains caseins and whey proteins in the same ratio as milk. Although ideally suited for delivery of nutritionally superior products, MPC has not been extruded into commercial puffed snacks nor breakfast cereals due to its heat sensitivity. Meanwhile, apple pomace (AP) is a marginally utilized by-product of juice production and is also a good source of fiber and phenolic compounds. Its co-extrusion with MPC using low-temperature, low-shear supercritical-fluid extrusion (SCFX) technology to develop high-protein, shelf-stable products offers an economically and nutritionally attractive opportunity. In this study, SCFX was used to develop expanded products from MPC with added AP and sugar for direct consumption. The expansion ratios, density, porosity, water diffusion coefficient, color, mechanical properties and total phenolic content of the MPC extrudates containing three levels of AP (5%, 15% and 25%) and sugar (5%, 10% and 15%) were measured. The changes in texture, density and water activity of the MPC extrudates during storage were also evaluated. Two extruded commercial products were evaluated for comparison. The results showed that the MPC extrudates had low density, high porosity and good textural properties, which did not change during 9 months of storage. Adding AP at higher concentrations (≥15%) adversely affected the expansion characteristics of the extrudates. At lower AP concentrations, the texture of the extrudates was comparable to commercial products. As expected, the addition of AP and sugar caused the product to appear darker in color. Additionally, SCFX processing increased the availability of phenolic compounds in the extrudates, indicating improvements in the overall nutritional quality of the final product.