A simple, rapid, sensitive and accurate gas chromatography–mass spectrometric method was developed and validated for the simultaneous determination of hydroxyzine and cetirizine in whole blood. Solid-phase extraction procedure using Bond Elut LRC Certify II columns was used for the isolation of hydroxyzine and cetirizine from 1 mL whole blood followed by derivatization with a mixture of acetic anhydride:n-propanol (1:1, v/v). Limits of detection and quantification were 1.50 and 5.00 ng/mL, respectively. The assay was linear within the concentration range of 5.00–1000.0 ng/mL and the correlation coefficient was R2 ≥ 0.993 for both analytes. Absolute recovery was determined at three quality control concentration levels and was found to be at least 87.2% for both substances. Intra-day and inter-day accuracy values for both hydroxyzine and cetirizine were ranged from −1.2 to 3.8% and −2.7 to 2.0%, respectively, at the three concentration levels studied, whereas their respective intra-day and inter-day precision values were less than 9.9 and 6.5%, respectively, in terms of relative standard deviation (%RSD). The developed method was successfully applied for the quantification of hydroxyzine and cetirizine concentrations in whole blood, during the investigation of clinical cases where these two antihistamines were detected.