Understanding the onset mechanism of flash sintering is essential for advancing electric-field-assisted sintering technologies. Herein, the onset temperature of flash sintering (FS) was examined for alumina–8 mol% yttria-stabilized zirconia (Al₂O₃–8YSZ) composites with varying molar ratios of Al₂O₃ and 8YSZ under an applied electric field of 900 V/cm. The results show a composition-dependent variation in the onset temperature, which can be divided into three different regions on the basis of the Al₂O₃ content, each reflecting a different onset mechanism. In region I (0–62.5 mol%), the flash sintering behavior is dominated by 8YSZ owing to the internal electrochemical reaction driven by the electric field. In region II (62.5–80 mol%), flash sintering is determined by the percolation networks of 8YSZ, which offer conduction paths for current. In region III (80–99 mol%), isolated 8YSZ particles catalyze the flash sintering of Al₂O₃ through interfacial dielectric breakdown. These results highlight a composition-dependent transition in the onset mechanism of flash sintering: Composites with low Al₂O₃ contents exhibit defect-dominated flash sintering associated with 8YSZ, whereas those with high Al₂O₃ contents follow a thermally controlled mechanism. Thus, Al₂O₃ and 8YSZ exhibit distinct onset mechanisms during flash sintering.