A gold nanoparticle decorated functionalized nanocarbon black (Au@f-NCB) hybrid nanocomposite–modified molecular wire glassy carbon paste electrode (MW-GCPE) was constructed for sensitive determination of anticancer drug formestane (FMST) by square-wave adsorptive anodic stripping voltammetry (SW-AASV). The synthesized Au@f-NCB hybrid nanocomposite was characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and dispersive X-ray spectroscopy (EDX). The fabricated Au@f-NCB/MW-GCPE was found to have higher electrocatalytic activity towards the oxidation of FMST than bare GCPE. The developed electrochemical sensor allowed the ultrasensitive and selective detection of FMST with a linear range (0.02–5.2 µM), low limit of detection (7.14 nM), and high sensitivity (90.0 μAμM⁻¹ cm⁻²). The constructed Au@f-NCB nanocomposite modified MW-GCPE revealed good repeatability, reproducibility, and higher stability. Furthermore, the developed sensor has been shown to be highly sensitive for detecting FMST in human fluids, Lentaron depot ampoules, and patient plasma. In addition, electrosensing of FMST in the presence of the anti-cancer chemotherapy drug doxorubicin (DOX) was explored, and simultaneous detection of the two drugs was achieved.