Geometric structures and electronic properties of Lithium doped magnesium clusters, LiMgn (n = 2–11), were investigated by utilizing Crystal structure Analysis by Particle Swarm Optimization (CALYPSO) structural searching method with density functional theory (DFT) calculations. Theoretical calculations showed that the most stable isomers are three-dimensional structure from n ≥ 3, and appear a structural unit of the six-Mg triangular prism in their structures from n ≥ 8. Moreover, the Li atoms prefer to occupy the convex capped sites. Analyses of stabilities revealed that LiMg9 with high C4v symmetry possesses the highest stability and the doping of lithium atom makes the stabilities of magnesium clusters increase. Upon an analysis of charge transfer, charges transfer take place from Li to Mg atoms, and there exist strong sp hybridization between Li and Mg atoms with the increasing number of magnesium atoms. At last, further chemical bonding analysis indicated that there is a strong Li-Mg interaction in the LiMg9 cluster.