Nitrogen-doped carbon nanofibers (N-CNFs) were prepared in a one-step method. N-CNFs were grown on an activated carbon fiber (ACF) substrate using catalytic chemical vapor deposition. Acetonitrile served as the single source of carbon and nitrogen. An approximately 10-fold increase of N-content, relative to the ACF substrate, was achieved in the densely and uniformly grown N-CNFs/ACFs. The prepared material, used as adsorbents for cadmium (Cd) and lead (Pb) in water, exhibited a significantly large adsorption capacity of approximately 161 ± 5 and 88 ± 3 mg/g for Cd and Pb, respectively, attributed to the lone pair of electrons in N atoms, which facilitated the formation of coordinate covalent bonds with the metal ions. The one-step method of preparing the N-enriched multiscale web of carbon micronanofibers in this study is simple, and the prepared material may be used as an efficient adsorbent for the removal of toxic metal ions from wastewater.