Magnesium (Mg) alloys are increasingly recognized as a promising material for the next generation of implants due to their biocompatibility, favorable mechanical strength, and ability to biodegrade effectively in physiological environments. However, their clinical utility is hindered by rapid corrosion. This study introduces and investigates the application of an ultrathin, ultrasmooth, and corrosion-resistant nitrogen-doped amorphous carbon (a-C:N) thin film on a magnesium alloy (Mg-0.5Zn-0.2Ca) for the first time. The a-C:N film was synthesized using a polymer composite based on branched polyethyleneimine and subsequently applied to the magnesium alloy surface to enhance its corrosion resistance. Comprehensive characterization using advanced techniques confirmed the amorphous nature of the synthesized film, revealing the presence of , , and C-N bonds. AFM analyses and electrochemical corrosion tests demonstrated that the synthesized a-C:N film exhibits excellent corrosion resistance and reduces the corrosion rate of the substrate. Additionally, cytotoxicity tests indicated that the film is non-toxic and compatible for orthopedic implant applications, thereby expanding the potential clinical use of Mg-based implants. Carbon, being a biocompatible and inert nonmetallic element, makes it a suitable choice for enhancing the biocompatibility and corrosion resistance of Mg-based implants.