Heteroatom-doped carbon dots (CDs) with a high photoluminescent quantum yield (PLQY) have recently attracted attention due to their applications in chemical sensors, photocatalysis, bioimaging, and drug delivery. Nitrogen and phosphorus are in close proximity to carbon in the periodic table and are key tracking elements in the field of biomedical imaging. These two elements alter the optical and electronic properties of CDs and help improve the fundamental understanding of their PLQY. This can also lead to multifunctional usage in photoimaging and photothermal therapy. However, most PLQYs resulting from the synthesis of P-doped CDs are currently below 50%. These CDs have limited usefulness in the fields of bioimaging and drug delivery. In this study, a single-step, high-efficiency hydrothermal method was applied to synthesize nitrogen and phosphorous-doped carbon dots ((N, P)-CDs) with a PLQY of up to 53.8% with independent emission behavior. Moreover, the CDs presented high monodispersity, robust excitation-independent luminescence, and stability over a large pH range. Spectroscopic investigations indicated that the PLQY of the (N, P)-CDs was primarily due to the addition of P and the passivation effect of the oxidized surface. The excellent fluorescence properties of (N, P)-CDs can be effectively and selectively quenched by Hg²⁺ ions. Such systems show a linear response in the 0–900 nM concentration range with a short response time, indicating their potential for applications in the fields of chemistry and biology.