Design and manufacturing engineers often focus on technical performance and cost as primary factors in the design of components and assemblies. With a changing market and a growing list of regulations, however, manufacturing decision makers must address economic, environmental, and social aspects of sustainability in product design and manufacturing. Current methods and tools for conducting such sustainability assessments often consider corporate level metrics and lack the detail necessary to assist engineering decision making at the product and process level. A sustainability assessment method is presented that takes advantage of unit process modeling and life cycle assessment (LCA) approaches to assist in product assembly design. The combination of these two approaches facilitates process level sustainability assessment by addressing unit manufacturing and assembly operations, while identifying relevant economic, environmental, and social metrics. This method is demonstrated to evaluate alternate product and process designs for an aircraft-like assembly.