With the advantages of eco-friendliness, low cost, and low density, Mg₂(Si,Sn) solid solutions are promising candidates for thermoelectric applications. In this work, Sb-doped Mg₂Si_0.4Sn_0.6 bulks were prepared with a combined method of solid-state reaction and high pressure synthesis, followed by spark plasma sintering. Our investigations show that Sb doping optimizes the carrier concentration, while Si/Sn alloying effectively suppresses the lattice thermal conductivity and induces a convergence of the two lowest-lying conduction bands. Additionally, numerous coherent Sn-rich nanoprecipitates are formed within micron-sized grains. All these factors contribute synergistically to improving the thermoelectric properties of Mg₂Si_0.4Sn_0.6. The optimal Mg₂(Si_0.4Sn_0.6)_0.985Sb_0.015 exhibits a power factor higher than 4 000 μW·m⁻¹·K⁻² and a lattice thermal conductivity less than 0.8 W·m⁻¹·K⁻¹ at temperatures higher than 600 K, leading to the highest ZT of 1.61 at 823 K. Current work demonstrates an effective approach to enhancing the thermoelectric performance of n-type Mg₂X solid solutions through doping, alloying, and microstructure modification.