The creep response, mechanical properties, and microstructure evolution of the Al−Zn−Mg−Cu alloy were investigated under different initial heat treatment conditions. The results indicate that the density of geometrically necessary dislocations (GNDs) increases during the initial creep stage (<0.5 h) and undergoes dynamic changes in the stable creep stage. During creep aging, the dislocation distribution within the grains becomes more uniform, and additional subgrains are formed. The key factors influencing creep behavior are crystal orientation and the degree of initial precipitation. Grains oriented in the <001> and <101> directions are more susceptible to deformation during the creep process. Based on a strength model, the inhibitory effects of the η' phase in T6 specimens and the GP I zone in T4 specimens on dislocation motion were evaluated. This study demonstrates that selecting an appropriate initial precipitation state is an effective strategy to enhance the creep aging response and to produce high-performance components.