LiNi0.8Co0.1Mn0.1O2 is considered as a promising cathode material for lithium ion batteries because of its high capacity and low cost. However, the LiNi0.8Co0.1Mn0.1O2 suffers structural instability and irreversible phase transition during charge/discharge processes, especially under high voltage, resulting in serious capacity fading and thermal runaway. Here, we propose a simple and effective method of modifying LiNi0.8Co0.1Mn0.1O2 by Mg doping. Benefiting from the pillaring effects of inactive Mg in the crystal structure, Li(Ni0.8Co0.1Mn0.1)1-xMgxO2 materials exhibit low Li+/Ni2+ cation mixing, high structural stability, and improved cyclic stability in the voltage of 3.0–4.5 V. The optimal Li(Ni0.8Co0.1Mn0.1)0.97Mg0.03O2 achieves a high capacity retention of 81% over 350 cycles at 0.5 C and exhibits enhanced thermal stability at 4.5 V. The promotion mechanism is explored systematically by a combination study of electrochemical characterizations, demonstrating the faster Li+ diffusion kinetics, higher electronic conductivity, and stronger structure due to the Mg doping. Moreover, the full cell of Li(Ni0.8Co0.1Mn0.1)0.97Mg0.03O2//mesocarbon microbeads delivers a promising energy density of 595.3 W h kg−1 at 0.5 C (based on the mass of the cathode). The present work demonstrates that moderate Mg doping is a facile yet effective strategy to modify high-performance LiNi0.8Co0.1Mn0.1O2 for high-voltage lithium ion batteries.