This article presents the fabrication of size-/shape-controllable monodispersed Fe–Ni alloy nanostructures via a facile solvothermal method by the co-reduction of Fe³⁺ and Ni²⁺ with hydrazine hydrate in a strong alkaline media. The products were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), energy dispersive X-ray spectroscopy (EDX) and vibrating sample magnetometry (VSM). The sizes and morphologies of the Fe–Ni alloy nanostructures are dependent on the reaction conditions, such as surfactant, solvent, the molar ratio of metal ions and the amount of alkali. Excitingly, the magnetic hysteresis measurements revealed that the FeNi₂ alloy nanostructures display ferromagnetic behavior with enhanced magnetic properties at room temperature. In addition, FeNi₂ alloy nanoparticles with different sizes were used as reusable heterogeneous catalysts to reduce p-nitrophenol (p-NP) to p-aminophenol (p-AP) by NaBH₄ in aqueous solution and show a good potential application in the area of chemical catalysis. Furthermore, the Fe–Ni alloy nanoparticles also have potential applications in the fields of magnetic recording devices, biotechnology and environmental treatment.