The addition of Sc and Zr to 5xxx5xxx aluminium alloysAluminium alloys significantly improves their mechanical performance through the formation of Al₃Al₃(Sc,Zr) core–shell precipitation(Sc,Zr) precipitates with a core–shell morphology. This study investigates how varying Mg content affects the precipitation behaviour in Al–Mg–Sc–Zr alloys. ThermodynamicThermodynamics modelling was used to optimise the solution treatment conditions to maximise solute retention while avoiding incipient meltingMelting. A combination of in-situ and ex-situ techniques, including hardnessHardness, conductivity, atom probe tomographyAtom probe tomography, and small-angle X-ray scattering, was used to examine the precipitation sequence. Special attention is given to the development of core–shell morphologies, which are critical for long-term thermal stabilityThermal stability. The findings highlight that Mg slows down early-stage precipitation kinetics while accelerating shell formation at later ageing stages. These effects are discussed in terms of Mg’s influence on Si solute behaviour, known to promote nucleation of Al3Sc, and on interfacial energyEnergy. These insights are key to the design of next-generation Al–Mg–Sc–Zr alloys.