We report here the fast fabrication of fluorescent carbon dots (CDs) within several minutes by magnetic hyperthermia. This method offers an alternative platform to prepare excitation-dependent fluorescent CDs without any surface modification from various kinds of precursors varying from polymers like poly(vinyl alcohol) (PVA) and polyethylene glycol (PEG) to small organic molecules and salts like amino acid, glucose, EDTA, citrate acid-urea, sodium citrate, zinc stearate, and ammonium citrate. As an example, the magnetic hyperthermia synthesis, characterization, optical properties, and in vitro cytotoxicity of CDs obtained from ammonium citrate are thoroughly investigated. The excitation-dependent fluorescence mechanism is discussed based on the UV-vis absorption, excitation, and emission spectra as well as the photoluminescence (PL) response to metal ions and pH. These CDs are further used as ink for inkjet printing to produce fluorescent patterns with uniform and stable PL useful for anticounterfeit and optoelectronic applications. This magnetic hyperthermia method allows the rapid fabrication of versatile CDs from a wide range of carbon sources in a simple, low-cost, and easy-to-perform way.