We present a green synthesis of fluorescent carbon quantum dots (CQDs) by using red cabbage (rc) and a one-step hydrothermal approach. The rcCQDs were characterized by various techniques such as UV-visible spectroscopy, transmission electron microscopy (TEM), high-resolution TEM, Fourier-transform infrared spectroscopy, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy. The synthesized rcCQDs exhibited an average size of 3 nm, measured by TEM, blue fluorescence, and a quantum yield of 8.3%. The highest emission band was observed at approximately 402 nm when excited at 330 nm. The rcCQDs exhibited strong antioxidant activities by scavenging 61, 56, and 91% against 2, 2-diphenyl-1-picrylhydrazyl, hydroxyl, and potassium permanganate radicals, respectively. The scavenging activity of rcCQDs was comparable with that of standard antioxidant L-ascorbic acid. Cell Counting Kit (CCK)-8 assay depicted superior bio-compatibility and negligible cytotoxicity of rcCQDs on SH-SY5Y neuroblastoma cells. They were used as a fluorescent probe for bio-labeling ofEscherichia coliandStaphylococcus aureus. The viabilities of the labeled bacterial cells were analyzed by AFM and UV-visible spectroscopy. Furthermore, the rcCQDs were utilized as a fluorescent ink, an alternative to pens, and maybe suitable for paints and varnish agents. This study provides detailed mechanistic insights into the antioxidant activity of as-synthesized rcCQDs, which suggest the practical applicability of CQDs for bio-medical applications.