Efficient extraction is a vital step in mercury speciation. In this context, using 4-vinylbenzeneboronic acid and 9-vinylanthracene as functional monomers, a new magnetic adsorbent was fabricated according to one-pot hydrothermal approach. Various characterization results prove the as-prepared adsorbent presented abundant functional groups and saturation magnetism. Combining with magnetic solid phase extraction (MSPE), the adsorbent exhibited satisfactory entrapment performance towards different mercury species which had been pre-coordinated with dithizone to form metal-organic coordination. A series of parameters influencing the extraction performance were inspected in detail. Under the most beneficial conditions, sensitive and reliable approach to quantify trace methylmercury, ethylmercury, phenylmercury and inorganic mercury in aqueous samples was developed by the combination of HPLC/DAD. Limits of detection and precision located in the ranges of 0.012–0.074 µg/L and 2.5–9.8%, respectively. Recoveries with low, medium and high fortified contents in actual waters varied from 79.8 to 119%. Confirmatory experiments were performed to evidence the accuracy and reliability of established approach. In addition, a possible mechanism was suggested based on the chemical nature of analytes, extraction conditions and characterization results.