An integrated drive system assembly is provided that combines an electric motor, a power inverter assembly and a gearbox into a single, multi-piece enclosure. Combining these components into a single enclosure reduces weight, reduces drive system complexity, reduces system volume, simplifies assembly integration into an electric vehicle, reduces manufacturing cost, allows the flexible and lengthy electrical cables between the power inverter and the electric motor to be replaced with short, low loss, rigid bus bars, and simplifies component cooling by allowing the use of a common thermal management system, i.e., a mutual thermal management system. The common thermal management system includes a liquid coolant loop that is thermally coupled to the electric motor, the power inverter assembly and the gearbox.