In recent years, wire-arc directed energy deposition (waDED), which is also commonly known as wire-arc additive manufacturing (WAAM), has emerged as a promising new fabrication technique for magnesium alloys. The major reason for this is the possibility of producing parts with a complex geometry as well as a fine-grained microstructure. While the process has been shown to be applicable for Mg-Al-Zn alloys, there is still a lack of knowledge in terms of the influence of the WAAM process on the age-hardening response. Consequently, this study deals with the aging response of a WAAM AZ91 alloy. In order to fully understand the mechanisms during aging, first, the as-built condition was analyzed by means of high-energy X-ray diffraction (HEXRD) and scanning electron microscopy. These investigations revealed a fine-grained, equiaxed microstructure with adjacent areas of alternating Al content. Subsequently, the difference between single- and double-step aging as well as conventional and direct aging was studied on the as-built WAAM AZ91 alloy for the first time. The aging response during the various heat treatments was monitored via in situ HEXRD experiments. Corroborating electron microscopy and hardness studies were conducted. The results showed that the application of a double-step aging heat treatment at 325 °C with pre-aging at 250 °C slightly improves the mechanical properties when compared to the single-step heat treatment at 325 °C. However, the hardness decreases considerably after the pre-aging step. Thus, aging at lower temperatures is preferable within the investigated temperature range of 250-325 °C. Moreover, no significant difference between the conventionally aged and directly aged samples was found. Lastly, the specimens showed enhanced precipitation kinetics during aging as compared to cast samples. This could be attributed to a higher amount of nucleation sites and the particular temperature profile of the solution heat treatment.