4-Chloro-2-methylphenoxyacetic acid (MCPA) is one of the pesticides most widely used to control broadleaf weeds in arable and horticultural crops and it leaches readily into groundwater bodies causing pollution. In this study, a sensor was fabricated by the simple activation of a glassy carbon electrode (GCE) in an aqueous phosphate solution by cycling the GCE between –2.0 and 2.4 V vs SCE. The activated GCE exhibited very good detection of MCPA, with a linear concentration range extending from 1 to 850 μM (cyclic voltammetry) and a limit of detection (LOD) of 0.008 μM, which was obtained using differential pulse voltammetry. A more thermodynamically favoured oxidation of MCPA was observed at the activated GCE, with an approximate shift of 110 mV in the peak potential to lower potentials. Very good reproducibility and stability were achieved, with the sensor giving similar peak currents over a 30-day immersion period. Good selectivity was achieved in the presence of nitrates, nitrites, sulfates and structurally related compounds such as nitro-phenols. The promising performance of the activated GCE in the sensing of MCPA was attributed to the generation of oxygenated functional groups and an increased surface area arising from the local dissolution of the GCE during activation.