This study analyzed the effect of high-energy milling (HEM) and subsequent isothermal annealing on the microstructural characteristics of a premixed Al-Zn-Mg-Cu powder, Alumix 431D, with a chemical composition similar to 7075 Al alloy. To this end, the mentioned powder was mechanically milled for 14 h using a horizontal attritor mill. Then, the milled powder was isothermally annealed at temperatures ranging from 200 °C to 500 °C for 1 h up to 4 h in an Ar atmosphere. HEM caused the T-Mg32 (Al, Zn)49 phase present in the microstructure of Alumix 431D powder particles to dissolve into their ?-Al solid solution matrix. In addition, the crystallite size of the ?-Al phase decreased to 30 nm through HEM. The nanostructured 7075 Al alloy processed by HEM showed good thermal stability and resistance to grain growth, retaining its crystallite size of less than 100 nm after the annealing of milled Alumix 431D powder at 400 °C for 4 h.