Binary alloy phase diagrams at high pressure are essential for understanding solidification and chemical partitioning in both engineered materials and planetary interiors, yet their experimental determination becomes increasingly challenging under extreme conditions. We apply a fully ab initio approach [J. Chem. Phys. 162, 184502 (2025)], based on density-functional theory, to compute the dilute-limit phase diagram of the Al–Mg system from ambient conditions up to 150 GPa. As a first step, we calculate the melting curves of pure fcc Al and pure hcp/bcc Mg, including the hcp–bcc phase boundary and triple point of Mg, all of which agree closely with available experimental data. The binary phase diagram is then constructed at both compositional extremes. On the Al-rich side, Mg consistently favours the liquid throughout the entire pressure range, with this preference strengthening monotonically under compression. On the Mg-rich side, Al partitions preferentially into the liquid at ambient pressure but undergoes a complete reversal within the bcc stability field of the Mg host, becoming solid-favouring (k = 1.12 ± 0.03) by 60 GPa. This pressure-driven reversal inverts the topology of the Mg-rich coexistence field from a conventional downward-sloping to an upward-sloping phase boundary, and can be useful for models of planetary differentiation and interior chemical stratification.