A hydrophobic/cation-exchange polymer monolith was prepared via one-step thermally initiated polymerization of 2-acrylamido-2-methyl-1-propyl-sulfonic acid (AMPS), divinylbenzene (DVB) and ethylene glycol dimethacrylate (EDMA) in a capillary. The use of DVB and EDMA as binary crosslinking monomers help to increase the specific surface area and enhance hydrophobicity of the target monolith. The as-obtained monolith was characterized by diffuse reflectance infrared spectroscopy, scanning electron microscopy, nitrogen adsorption–desorption and mercury intrusion porosimetry. The results show that the monolith has favorable permeability and well mechanical strength. Furthermore, its specific surface area is up to 353 m² g⁻¹. The as-prepared monolith was used as a sorbent for polymer monolith microextraction (PMME), which was coupled to high performance liquid chromatographic-electrospray ionization-mass spectrometric (HPLC-ESI-MS) analysis in off-line mode for the determination of antidepressants in biological samples. The results show that the monolith with hydrophobic and strong cation-exchange functional groups exhibits high extraction efficiency towards the antidepressants. The limits of detections (S/N = 3) for the antidepressants in plasma samples were in the range of 0.06–0.39 ng mL⁻¹ and the recoveries were from 73.2% to 110.8% (depending on the analytes), with relative standard deviations (RSDs) less than 9.8%.