We present the 5-18 micron spectrum of brown dwarf WISE-J1828, observed with the MIRI Medium Resolution Spectrometer (MRS) on JWST. This is the first mid infrared spectrum of a cold Y-dwarf (T~400 K), where the spectral range covers the peak of its emission. At the resolution of the MRS the spectrum shows beautiful spectral features of various molecules, most notably ammonia. We discuss the data processing, spectral extraction, and first science results of this unique dataset, highlighting the detection an isotopologue of ammonia. The fidelity and richness of the spectrum will establish it as the benchmark for future studies of cool objects, opening a new window towards understanding atmospheric models of brown dwarfs, but also of cool giant exoplanets.