Abstract: A hanger installing tool includes a pair of slidably mounted posts and an impact member for the driving of prongs of a joist hanger into a header. Arm members of the tool position the tool vertically on a header side with retainers on the arm members confining the tool in header engagement. Limit stops on the tool position an inserted hanger into place on the tool. A bracket attachable to the tool provides one of multiple limit stops for hangers of various lengths. A shoulder on the tool is abutted by the upper end of a hanger to contribute to proper hanger installation. Opposed surface of the tool are engaged in a biased manner by angular flanges of the hanger providing hanger retention during header attachment. Threaded sockets permit selective attachment of a tool handle to best serve convenient use of the tool.