Problematic biofilms cause microbiologically influenced corrosion (MIC) and biofouling in many industries such as the oil and gas industry, water utilities, and the power generation industry. Equipment failures cause not only economic losses but also environmental damages. A high biocide concentration is required to treat mixed-culture biofilms due to their various defense mechanisms. A biocide enhancer can reduce the dosage or make a biocide more effective. In this work, an equimolar mixture with 100 ppm (w/w) of four D-amino acids (D-tyrosine, D-methionine, D-leucine, and D-tryptophan) labeled as D-mix enhanced 60 ppm alkyldimethylbenzylammonium chloride (ADBAC) against a field biofilm consortium on C1018 carbon steel coupons by achieving at least an extra 2-log reduction of sessile cell counts in the biofilm prevention test. Scanning electron microscopy images, confocal laser scanning microscopy images, weight loss, and pitting data all corroborated the electrochemical tests.