The microstructure and mechanical properties of the WAAM-ZL205A alloy were investigated at various thermal exposure temperatures and compared with those of the deposited and T6-treated alloys. Due to the precipitation strengthening of fine and scattered θ′′ and θ′ phases, the yield strength (YS) of the T6-treated ((535 °C, 10 h) + (175 °C, 10 h)) alloy increases to 366.9 MPa compared with that of the deposited alloy. However, the elongation (El) is reduced to 12.9%. When the thermal exposure temperature increases from 200 to 300 °C, there is a notable decrease in YS and the time required to stabilize is shortened. After thermal exposure at 300 °C for 24 h, part of the θ′ precipitate located at the grain boundary transforms into the θ phase, while the θ′ precipitate within the grains experiences coarsening, resulting in a decrease in YS to 188.9 MPa. Upon surpassing an exposure duration of 120 h, the mechanical characteristics exhibit stability.