•The cryogenic magnetocaloric effect in different magnetic systems is discussed.•The assembly strategies toward 3d-, 4f- and 3d–4f based molecular magnetic coolants are described.•The recent advance in cryogenic molecular magnetic coolants is summarized.•The different nature of 3d and 4f ions must be considered in the design of molecular magnetic coolants. This review outlines recent advances in the design of 3d-, 4f-, and 3d–4f type magnetic molecules for use as excellent cryogenic magnetic coolants based on the magnetocaloric effect (MCE), and the structure-magnetocaloric correlations of reported molecular coolants. Further improvements in the MCE values of molecular magnetic materials are also proposed based on assembly strategies from molecular chemistry and crystal engineering.