L-lysine amino acid is cocrystallized with L-mandelic acid by the slow evaporation method. Single crystal X-ray analysis reveals that lysine-mandelic acid crystallized as a dihydrate form. In the crystalline state, the lysine molecule exists in the cationic form in which the backbone and side chain amino groups are protonated and the carboxylic acid is deprotonated. The carboxylic acid proton of the mandelic acid is transferred to the lysine side chain and thus carries a negatively charged ion. The lattice water molecules are located near the amino groups of the lysine. Intermolecular interactions formed between lysinium, mandelate and lattice water molecules are qualitatively analyzed using Hirshfeld surfaces and 2D-fingerprint plots. The energetics of different dimeric complexes is quantitatively analyzed using PIXEL energy analysis. Topological parameters derived from QTAIM framework are used to delineate the nature of different intermolecular interactions formed in the title complex.