Due to the coexistence of many emergent phenomena, including 2D superconductivity and a large Rashba spin-orbit coupling, 5d transition metal oxides-based two-dimensional electron systems (2DESs) have been prospected as one of the potential intrants for modern electronics. However, despite the lighter electron mass, the mobility of carriers, a key requisite for high-performance devices, in 5d-oxides devices remains far behind their 3d-oxides analogs. The carriers' mobility in these oxides is significantly hampered by the inevitable presence of defects. Here, very high mobility (approximate to 22 650 cm(2) V-1 s(-1)) of 5d-2DES confined at the LaAlO3/KTaO3 interface is reported. The high mobility, which is beyond the values observed in SrTiO3 2DESs in the same carrier-density range, is achieved using the ionic-liquid gating at room temperature. The authors postulate that the ionic-liquid gating affects the oxygen vacancies and efficiently reduces any disorder at the interface. Investigating density and mobility in a broad range of back-gate voltage, the authors reveal that the mobility follows the power-law mu proportional to n(1.2), indicating the very high quality of ionic-liquid-gated LaAlO3/KTaO3 devices, consistent with the postulate. Furthermore, the analysis of the quantum oscillations confirms that the high-mobility electrons occupy the electronic sub-bands emerging from the Ta:5d orbitals of KTaO3.