A carbon-paste modified electrode decorated with La2O3 (LaOx/CPE) is reported for the first time for the voltammetric determination of thiomersal (THM) content. The LaOx/CPE surface was characterized using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The presence of La2O3 on the surface of the carbon paste reduces the charge transfer resistance and improves the sensitivity toward the oxidation of THM, with an oxidation current increase of almost 60% and allows its determination by square-wave stripping voltammetry (SWSV). The detection limit (3σ/b) was 0.09 μmol L−1 and the relative standard deviation was 2.5% (n = 7) with three different electrodes. Furthermore, the new sensor was applied in the detection of THM in vaccines and pharmaceutical dosages.