A single-laboratory validation study was undertaken for the analysis of paralytic shellfish poisoning (PSP) toxins in common mussels, extending AOAC Official Method 2005.06 to include the additional toxins dcNEO and dcGTX2,3. The method was refined to improve toxin oxidation product stability, analytical sensitivity of N-hydroxylated toxins, and throughput. Validation was performed to characterize the method for selectivity, sensitivity, linearity, precision, repeatability, recovery, ruggedness, and uncertainty. Parallel testing of naturally contaminated mussels enabled comparison of sample toxicities obtained using mouse bioassay (MBA) and high-performance liquid chromatographic (HPLC) methodologies. Performance characteristics of the method are reported for all commercially available certified reference toxins. Results from the MBA and HPLC methods were well correlated, and the analytical method has been instigated as the sole monitoring tool for UK official control surveillance of PSP toxins in common mussels.