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 through an access port of the gas turbine engine such that the repair tool includes a tip end positioned within the gas turbine engine and a material supply end 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 of the gas turbine engine, wherein the defect defines a fillable volume along a portion of the internal component. In addition, the method may include supplying a filler material to the tip end of the repair tool and expelling the filler material from the tip end of the repair tool such that the fillable volume is at least partially filled with the filler material.