A simple and reliable method using liquid chromatography with diode array detector was developed for the simultaneous determination of florfenicol and thiamphenicol in medicated feed. The analytes were extracted from the minced feed with methanol and ethyl acetate (1:1, v/v). Next, the extract was further cleaned up by dispersive solid phase extraction using anhydrous magnesium sulfate, PSA and C18 sorbents. Finally, 1 mL of extract was evaporated, the residue resuspended in Milli-Q water, and filtered. The method was validated in-house at medicated levels, in the concentration range 10–300 µg/mL (50–1500 mg/kg). Values of <6.5% and <6.0% were found, respectively, for repeatability and within-laboratory reproducibility. The LODs for the two fenicols were 2.4–5.3 mg/kg, while the LOQs were 3.8–5.6 mg/kg. The expanded uncertainty was estimated to be in the range of 10.0–14.5%, depending on the analyte. Recoveries varied from 81.7% to 97.5%. The methodology was applied to the analysis of animal feedingstuffs collected from poultry and pig farms.