A through tubing centralizer is delivered below the tubing and expanded against a surrounding tubular for fixation. In a window milling through tubing application, a whipstock is delivered through tubing and anchored. One or more centralizers are then delivered through tubing and expanded so that their inside diameter when set exceeds their outside diameter during run in. They are placed below the tubing and above the whipstock to help a through tubing mill stay on the whipstock ramp while milling the window. After the window is started, the centralizer can be expanded fully against the surrounding tubular to allow subsequent removal of the whipstock. Optionally, the centralizer can be collapsed for removal through tubing.