The demand for the development of fast, reliable, and cost-effective screening methodologies for pesticide residues is critical. To meet the requirements of on-site monitoring purposes, electrochemical sensors are superior tools. Herein, this work describes the fabrication of a facile, disposable, easy-to-use, and enzymeless sensor platform for the determination of an organophosphorus pesticide, fenitrothion (FNT), in real samples. In this sense, electrosynthesis of poly-arginine (p(Arg)) on graphene oxide (GO) functionalized disposable sensor and its utilization for the voltammetric detection FNT were evaluated for the first time. p(Arg) film was grown onto GO-modified pencil graphite electrode by employing easy, practical, and an eco-friendly electro-polymerization route via cyclic voltammetry using electroactive groups in arginine structure. The p(Arg)-supported GO nanocomposite was further characterized using spectroscopic, microscopic, and electrochemical analyzes. The fabricated (p(Arg)-GO)-based sensor was applied as a recognition element to acquire the direct quantitative electroanalysis of FNT by monitoring its irreversible reduction peak. The combination of p(Arg) and GO led to high affinity, therefore, synergistically contributed to an electrocatalytic effect toward FNT detection in the presence of some interferents. Square wave voltammetry allowed rapid response to reach a linear calibration range as 0.69–20.2 μM for FNT detection under the optimum conditions and the limit of detection was estimated as 0.173 μM. Finally, the assessment of the suggested non-enzymatic electrochemical nanosensor for cucumber and water samples displayed attractive practical ability with satisfactory recovery results making the method useful for the determination of FNT in real matrices. Such p(Arg)-GO-based sensing platform holds considerable promise toward on-site determination of pesticides as a portable decentralized testing system.