Ammonium phosphomolybdate,  anion, was reported in 1826. The isostructural phosphotungstate anion was characterized by X-ray crystallography 1934. This structure is called the Keggin structure after its discoverer.
The 1970's witnessed the introduction of quaternary ammonium salts of POMs. This innovation enabled systematic study without the complications of hydrolysis and acid/base reactions. The introduction of O NMR spectroscopy allowed the structural characterization of POMs in solution.
Ramazzoite, the first example of a mineral with a polyoxometalate cation, was described in 2016 in Mt. Ramazzo Mine, Liguria, Italy.