Monodisperse 4 nm FePt magnetic nanoparticles were synthesized by superhydride reduction of FeCl2 and Pt(acac)2 at high temperature, and thin assemblies of FePt nanoparticles with controlled thickness were formed via polymer mediated self-assembly. Adding superhydride (LiBEt3H) to the phenyl ether solution of FeCl2 and Pt(acac)2 in the presence of oleic acid, oleylamine, and 1,2-hexadecanediol at 200 °C, followed by refluxing at 263 °C, led to monodisperse 4 nm FePt nanoparticles. The initial molar ratio of the metal precursors was retained during the synthesis, and the final FePt composition of the particles was readily tuned. Alternately, adsorbing a layer of polyethylenimine (PEI) and the FePt nanoparticles onto a solid substrate resulted in nanoparticle assemblies with tunable thickness. Chemical analysis of the assemblies revealed that more iron oxide was present in the thinner assemblies annealed at lower temperature or for shorter time. Thermal annealing induced the internal particle structure change from chemically disordered fee to chemically ordered fct and transformed the thin assembly from superparamagnetic to ferromagnetic. This controlled synthesis and assembly can be used to fabricate FePt nanoparticle-based functional devices for future nanomagnetic applications.