Our bodies contain a neurotransmitter called serotonin, which regulates several activities connected to our temporary state of mind or temper. Serious disorders linked to mood disorders, anxiety, and depression are diagnosed due to serotonin disturbance. If diagnosed early, serotonin fluctuations could be controlled through proper medication. We present the electrochemical detection of serotonin molecules at the femtomolar level using carboxylate molybdenum disulfide (MoS2-COOH) functionalized on the gold electrode surface. X-ray diffraction spectroscopy explores the synthesized MoS2-COOH conjugates structural properties, and their functional behavior is confirmed by FT-IR study. The square wave voltammetry confirmed that the carboxylated MoS2 nanocomposite-modified electrode offered excellent electrocatalytic activity towards serotonin oxidation. The sensor excels in sensing serotonin over a wide dynamic range from 1 µM to 1 fM, with a limit of detection of 12.038 fM, excellent reproducibility, and high selectivity towards serotonin in interfering medium containing dopamine. This sensitive, selective, and cost-effective electrochemical sensor shows significant potential in direct serotonin analysis for clinical applications.