Geotechnical design engineers used to rely on arbitrary rules and definitions of “factor of safety” on peak soil strength in limit analysis calculations. They used elastic stiffness for deformation calculations, but the selection of equivalent linear elastic models was always arbitrary. Therefore, there is a need for a simple unified design method that addresses the real nature of serviceability and collapse limits in soils, which always show a nonlinear and sometimes brittle response. An approach to this method can be based on a new application of the theory of plasticity accompanied by the introduction of the concept of “mobilizable soil strength.” This approach can satisfy both safety and serviceability and lead to simple design calculations within which all geotechnical design objectives can be achieved in a single step of calculation. The proposed method treats a stress path in an element, representative of some soil zone, as a curve of plastic soil strength mobilized as strains develop. Designers enter these strains into a plastic deformation mechanism to predict boundary displacements. The particular case of a cantilevered retaining wall supporting an excavation in clay is selected for a spectrum of soil conditions and wall flexibilities. The possible use of the mobilizable strength design (MSD) method in decision-making and design is explored and illustrated.