Per- and polyfluoroalkyl substances (PFAS) are persistent, bioaccumulative, and ubiquitous contaminants that are harmful to both humans and ecosystem health. To remove PFAS effectively and efficiently from the aqueous environment, a clay-based adsorbent was synthesized via the modification of montmorillonite by a cationic surfactant cetyltrimethylammonium chloride (CTAC). Through the coexposure adsorption tests with organic dyes and PFAS mixtures, the optimal ratio of CTAC to cation exchange capacity (CEC) was identified. The optimal modified clay exhibited drastically improved adsorption performance, achieving 100% removal efficiency of the PFAS mixture consisting of nine short- and long-chain PFAAs, GenX, and three precursors at initial concentrations of the parts per billion (ppb) level. Additionally, the modified clay outperformed other commercial adsorbents with respect to adsorption performance. The adsorption kinetic data of all PFAS, which were well described by the pseudo-second-order model, suggested an expeditious adsorption process and an adsorption behavior dependent on the initial concentration and carbon chain length. Among the three examined adsorption isotherms, the Sips model combining Langmuir and Freundlich models showed the best fitting correlation, indicating that multiple interactions might be involved in the adsorption process. This hypothesis was supported by characterizations showing that the modified clay possessed physicochemical properties favorable for electrostatic interactions and hydrophobic interactions.