ε-Iron carbide has garnered increasing interest for its superior magnetic characteristics and catalytic performance compared to other iron carbides. However, its metastable nature has posed significant challenges for synthesis, often requiring ultrahigh pressure, multistep processes, complex reaction condition control, and highly toxic reagents. Consequently, the properties of ε-iron carbide remain largely unexplored. A simplified synthesis method for ε-iron carbide can accelerate the exploration of new functionalities. In this study, a novel one-step selective synthesis method for ε-iron carbide nanoparticles under mild conditions via a wet-chemical approach is presented. In this method, Fe(CO), cetyltrimethylammonium bromide (CTAB), and bis(pinacolato)diboron (Bpin) are added to hexadecylamine and reacted at 220 °C-a simple process that eliminates the need for extreme pressures and toxic substances. Detailed investigations elucidate the crucial roles of CTAB and Bpin in facilitating the selective formation of ε-iron carbide. This accessible and efficient synthesis process for ε-iron carbide can further enable the discovery of unprecedented catalytic properties in the reductive amination of benzaldehyde, distinct from those of conventional iron nanoparticle catalysts. Density functional theory calculations reveal insights into the electronic states responsible for the distinct activity of the ε-iron carbide nanoparticles.