We show how replacing a fraction of Mg with Li in a lean Al-Mg-Si alloy gives comparable strength and enhanced temperature stability. Replacing solute with smaller amounts of Cu and Li also improves thermal stability and nearly compensates the strength loss for longer ageing times. The Cu addition was kept at 0.021 at. %, so that the formation of β″ type precipitates was not suppressed. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) documented Li and Cu causing modest structural changes to the main hardening precipitate, β″. However, density functional theory calculations verified the observation from HAADF-STEM that Li preferentially occupies Mg3 sites in the β″ structure. By substituting some Mg with Li, it is possible to achieve higher strength-to-weight ratios without altering the precipitation sequence of the alloys or sacrificing the hardening potentials.