In this study, a carbon paste electrode modified with silver and magnetite nanoparticles was used to quantify arsenic via square-wave anodic stripping voltammetry. Under optimized parameters, a deposition potential of −0.4 V for a period of 70 s, the setup achieved a linear quantification range from 0 to 40 µg L⁻¹ of As (III) (R² = 0.982). Electrochemical characterization was performed using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The evaluated arsenic quantification framework encompasses the World Health Organization (WHO) limit and the Mexican maximum permissible level for arsenic in drinking water (10 µg L⁻¹), as well as arsenic concentrations frequently reported in groundwater wells throughout northern Mexico. Key advantages of this electrode include its low fabrication cost, ease of operation, and analytical stability spanning a 30-day period. Additionally, selectivity tests revealed that Cu(II) masked the arsenic stripping signal at a 1:1 Cu(II): arsenic concentration ratio, whereas no modification of the target signal was observed in the presence of cadmium.