Potassium-ion batteries (KIBs) have attracted increasing attention for grid-scale energy storage due to the abundance of potassium resources, low cost, and competitive energy density. The key challenge for KIBs is to develop high-performance electrode materials. However, the exploration of high-capacity and ultrastable electrodes for KIBs remains challenging because of the sluggish diffusion kinetics of K+ ions during the charging/discharging processes. This study reports for the first time a facile ion-intercalation-mediated exfoliation method with Mg2+ cations and NO3– anions as ion assistants for the fabrication of expanded few-layered ternary Ta2NiSe5 (EF-TNS) flakes with interlayer spacing up to 1.1 nm and abundant Se sites (NiSe4 tetrahedra/TaSe6 octahedra clusters) for superior potassium-ion storage. The EF-TNS deliver a high capacity of 315 mAh g–1, excellent rate capability (121 mAh g–1 at a current density of 1000 mA g–1), and ultrastable cycling performance (81.4% capacity retention after 1100 cycles). Detailed theoretical analysis via first-principles calculations and experimental results elucidate that K+ ions intercalate through the expanded interlayers effectively and prefer to transport along zigzag pathways in layered Ta2NiSe5. This work provides a new avenue for designing novel ternary intercalation/pseudocapacitance-type KIBs with high capacity, excellent rate capability, and superior long-term cycling performance.