The lightweight, unconventional 2055 Al-Cu-Li-Ag alloy exhibits an excellent specific strength in the T83 state, but no literature reports the effects of overaging on this alloy. In the present work, the suitability of the alloy for lightweight components operating at high temperature is evaluated. Thermal exposure in the range 215–305 °C was investigated, highlighting its consequences on both microstructure and mechanical properties. In the most severe overaging state (24 h at 305 °C), the typical T₁ precipitates (Al₂CuLi) are dissolved, leading to the formation and coarsening of ϑ’ and Ω phases. In all overaging conditions, the alloy performance was superior or at least comparable to that of another third generation Al-Li alloy, AA2099, which is characterised by a slightly lower density and encouraging mechanical properties for high temperature applications. Compared to AA2099, the AA2055 alloy provides a higher specific strength (the basic requirement for mass savings) both in the T83 and in the most severe overaging state (24 h at 305 °C). This work highlights that AA2055 is a promising candidate for lightweight components operating up to 305 °C, and it lays the basis for high temperature tests of the alloy.