The effects of N2 incorporation in Ar/CH4 plasma on the electrochemical properties and microstructure of ultra-nanocrystalline diamond (UNCD) films are reported. While the electrical conductivity of the films increased monotonously with increasing N2 content (up to 25%) in the plasma, the electrochemical behavior was optimized for UNCD films grown in (Ar–10% N2)/CH4 plasma. Transmission electron microscopy showed that the main factor resulting in high conductivity in the films was the formation of needle-like nanodiamond grains and the induction graphite layer encapsulating these grains. The electrochemical process for N2-incorporated UNCD films can readily be activated due to the presence of nanographite along the grain boundaries of the films. The formation of needle-like diamond grains was presumably due to the presence of CN species that adhered to the existing nanodiamond clusters, which suppressed radial growth of the nanodiamond crystals, promoting anisotropic growth and the formation of needle-like nanodiamond. The N2-incorporated UNCD films outperformed other electrochemical electrode materials, such as boron-doped diamond and glassy carbon, in that the UNCD electrodes could sense dopamine, urea, and ascorbic acid simultaneously in the same mixture with clear resolution.