Abstract: High-temperature thermal exposure of Al alloys is used to characterize the real-world performance of components in high-temperature environments. In this study, thin-walled Al–Cu heat-resistant aluminum alloy structures were additively manufactured using a wire-arc directed energy deposition (WA-DED) process. A systematic analysis was conducted to investigate the microstructural changes and determine the cause of mechanical degradation under 300°C thermal exposure. Following T6 heat treatment (solution treatment: 520°C/6 h + 535°C/8 h; aging: 175°C/2 h + 150°C/6 h), the WA-DED 205C (Al–Cu–Mn–Ni–Zr) alloy exhibited a dense distribution of nano-sized θ″, θ′, Al₃Zr, and T (Al₂₀Cu₂Mn₃) phases within the grains, whereas the γ (Al₇Cu₄Ni) phase formed during solidification remained at the grain boundaries. Following thermal exposure at 300°C for 24 h, the majority of θ″ within the grains was supplanted by θ′; a subset of these θ′ subsequently transitioned to θ, while σ (Al₅Cu₆Mg₂) phase concurrently precipitated. As thermal exposure reached 100 h, the θ′ phase exhibited increased particle size alongside reduced number density, whereas the θ phase showed enhanced density and the σ phase was no longer present. The Al₃Zr and T phases within the grains, and the γ phase located at the grain boundaries, showed no significant changes in morphology or size during the thermal exposure process. Furthermore, after thermal exposure at 300°C for 24 h, the microhardness of the T6 condition specimen experienced a sharp decline from HV 149.6 to HV 98.2, and the yield strength (YS) dropped from 353 to 213 MPa. After 100 h of thermal exposure, the microhardness decreased to HV 91.8 and the YS dropped to 196 MPa. The tensile properties of the alloy were stabilized at a YS retention rate of 56%.