A simple synthesis of carbon quantum dots (CQDs) from green sources has attracted increasing attention nowadays. Here we report the synthesis of water-soluble fluorescent carbon quantum dots from mandarin juice by hydrothermal method. When excited at 364 nm, CQDs give maximum fluorescence at 462 nm. As verified from Transmission Electron Microscopy, obtained CQDs were very small, with an average diameter of 2.37 ± 0.42 nm. These very small-sized nanoparticles were dispersed to nematic liquid crystal mixture E7. As is known, particle size is a significant parameter for nanoparticle dispersed liquid crystal applications. The threshold voltage of pure and CQDs doped liquid crystals was derived from dielectric spectroscopy measurements, and it was observed that the threshold voltage increased with the increasing CQDs doping concentration. The nematic to the isotropic phase transition temperature of samples was investigated with DSC thermograms and POM. The phase transition temperatures gradually decreasing with increasing CQDs doping concentration were revealed.