High-Mg# andesites are thought to be derived from partial melting of subducting oceanic basalts. Slab thermal modeling has shown that this requires a young (<25 Ma), very shallow (flat) slab and/or high shear stresses along the slab surface. These conditions currently do not exist in the Aleutian arc, the typical locality of high-Mg# andesites. Using kinematic-dynamic thermal models of a subduction zone that include time-dependent age and subducting rate of the oceanic plate entering the trench since subduction initiation, we find that partial melting of the subducting oceanic basalts is restricted to the western Aleutians (from ∼174° W to 195° W), consistent with the widespread occurrence of high-Mg# andesites in the western Aleutian arc. Our modeling suggests that consideration of temporal and spatial evolution of slab age and subducting rate along the Aleutian island arc is fundamental to the genesis of high-Mg# andesites in the western Aleutians.