Synthesis of doped carbon dots (CDs) with enhanced fluorescence properties from green precursors has gained considerable attention due to their multipotential applications such as chemical as well as biosensing, photocatalysis, electrocatalysis, drug delivery, and bioimaging applications. Here, we have prepared highly fluorescent N,S-doped CDs (N,S-CDs) using a single natural precursor, the Giloy stem, by a hydrothermal method to explore their multipotential applications. These N,S-CDs were demonstrated as fluorescent probes for the selective detection of toxic organic pollutants that include 4-nitrophenol (4-NP) and congo red (CR) with limits of detection (LODs) of 380 and 62 nM, respectively. The fluorescence quenching observed in N,S-CDs was the result of the inner filter effect (IFE). The methods developed were used to detect 4-NP and CR in real samples, such as tap water and pond water, with recovery ranges from 93.88 to 103.45% and from 95.05 to 99.04%, respectively. Furthermore, these N,S-CDs were employed for colorimetric and smartphone-assisted detection of CR. The advantages of using a smartphone for the detection of analytes include naked-eye visualization of the probe-analyte interactions, the use of a portable and easy-to-handle device, and the elimination of the need for an expensive spectrophotometer. In addition to the sensing application, the N,S-CDs showed radical scavenging activity.