A multifunctional sensing platform based on bright blue-fluorescent nitrogen-doped carbon dots (N-CDs) has been ingeniously designed for the sensitive determination of Hg²⁺ and pH. The N-CDs were facilely fabricated via a one-step hydrothermal treatment of citric acid and folic acid with admirable merits including exceptional stability, low toxicity and distinguished biocompatibility. Intriguingly, the obtained N-CDs can be utilized to detect Hg²⁺ on the basis of the dynamic quenching effect, showing a linear range of 0 μM–400 μM as well as a detection limit of 0.124 μM. In addition, the obtained N-CDs revealed a significant emission enhancement with increasing pH from 4.0 to 10.0 and acquired a good linearity in the pH range of 6.8–7.8. Taking advantage of efficient analysis of Hg²⁺ and pH in aqueous solution, the as-synthesized N-CDs have been extended to the visualization biosensing platform to timely track Hg²⁺ and pH in living cells.