In one aspect, a method for performing in situ repairs of internal components of a gas turbine engine may generally include inserting a repair tool within an interior of the gas turbine engine such that a tip end of the repair tool is positioned within the gas turbine engine and an exterior end is positioned outside the gas turbine engine. The method may also include positioning the tip end of the repair tool adjacent to a defect of an internal component, wherein the defect defines a fillable volume along a portion of the internal component. In addition, the method may include intermixing two or more constituents of a repair agent within the repair tool at a mixing location defined within the gas turbine engine, and expelling the repair agent from the tip end such that the fillable volume is at least partially filled with the repair agent.