Per- and polyfluoroalkyl substances (PFAS) are emerging contaminants that have been used extensively as firefighting agents and in a wide range of commercial applications around the world. As many of the most-common PFAS components are surfactants, they readily accumulate at interfaces, a process that can govern their environmental fate. There are thousands of PFAS compounds, and they have nearly always been used as mixtures, so it is common to find many different PFAS components present together in the environment. Furthermore, the interfacial behavior of ionic PFAS can be strongly influenced by the presence of salts, with adsorption dependent on both the composition and concentration of salts present. Any predictions of PFAS interfacial behavior made without considering both the mixed nature of PFAS present, as well as the composition of the salts present, have the potential to be off by orders of magnitude. To date, models capable of making predictions of PFAS interfacial adsorption when both mixed PFAS and mixed salts are present have not been presented. The work described here addresses this need by extending a mass-action model developed previously by the authors to allow predictions in cases where complex combinations of mixed PFAS and mixed salts are present. Predictions of PFAS interfacial affinity for a range of PFAS mixture conditions and ionic strengths are verified using experimentally-measured surface tension data. The new model provides physically-realistic prediction of interfacial adsorption of a wide range of PFAS mixtures over a wide range of salt concentrations and compositions. The model is capable of predicting interfacial adsorption of ionic/nonionic PFAS mixtures in the presence of salts, and can also make predictions when there is competitive adsorption between different PFAS components, a common case in PFAS source zones where high concentrations of multiple components are present and in foam fractionation reactors.