A novel method was developed and single-laboratory validated for the determination of free pantothenic acid (vitamin B5) in a wide range of infant and adult fortified food products. The method combines simple sample preparation and chromatographic analysis using ultra-performance LC coupled to tandem MS with positive electrospray ionization. Pantothenic acid was quantified using [13C6, 15N2]-pantothenic acid as an internal standard. Calibration curves were linear between 0.08 and 1.2 microg/mL (r2 = 0.9998), and average recovery varied between 95 and 106%. The method exhibited overall RSD(r) of 1.1% and RSD intermediate reproducibility from 2.5 to 6.0% in infant formulas and cereals. Comparison of results between total and free pantothenic acid showed that the analysis of free pantothenic acid gave a good estimation of total pantothenic acid in the range of products analyzed. The method provides reliable free pantothenic acid results in a wide range of fortified foods (infant and adult nutritionals, cereal products and beverages), and shows good correlation with the microbiological method AOAC Official Method 992.07. It is a more selective, faster, and robust alternative to microbiological determination.