Precipitation-hardenable Mg-Sm-Nd-Zn-Zr alloy is a high-performance magnesium alloy with great potential for aviation applications. A more comprehensive understanding of the precipitation evolution will help fully exert its mechanical properties. The hardness initially rose and then fell with alloy aging to 100 h at 200 °C. A peak hardness of 84.1 HV was achieved at the aging time of 20 h, with a 39.7 % increase in hardness compared to the unaged alloy. The primary precipitates were the GP zones and β′ phase at the under-aged stage. As the aging time was extended to 20 h, a more abundant β′ phase precipitated in the matrix, and the alloy achieved the peak-aged stage. Eventually, the over-aging stage came with the aging process continuing, where the β′ phase coarsened severely and further transformed into the β1 phase. When the aging temperature was raised to 350 °C, even under a short aging time of 1 h, the coarse β1 phase formed rapidly, and the β phase could also nucleate and grow directly on the β1 phase. This work offers a guide for developing aging treatment processes for Mg-RE alloys.