We have investigated the structural, microstructural, interfacial, and surface morphological properties of Ba0.60Sr0.40TiO3 thin films Mg doped from 0 to 20 mol%. A strong correlation was observed between the films materials properties and the prior determined dielectric and insulating characteristics as a function of Mg doping. Non-textured polycrystalline films with a dense microstructure and abrupt film-Pt electrode interface were obtained after annealing at 750 degreesC for 30 min. Single phase solid solution films were achieved at Mg doping levels up to 5 mol%, while multiphased films were obtained for Mg doping levels of 20 mol%. Decreases in the films dielectric constant, dielectric loss, tunability and leakage current characteristics were paralleled by a reduction in grain size as a function of increasing Mg dopant concentration. Our results suggest that Mg doping serves to limit grain growth and is thereby responsible for lowering the dielectric constant from 450 to 205. It is suggested that Mg behaves as an acceptor-type dopant at the grain boundary and is responsible for the doped films low dielectric loss and good leakage current characteristics. Examination of the performance-property trade-offs advocates the 5-mol% Mg doped BST film to be an excellent choice for tunable microwave device applications.