A tool post assembly includes a tool post body adapted to be mounted on a machine tool, a carrier block coupled to and movable relative to the tool post body by means of a tenon-and-mortise structure, and a lever pivotally mounted on the tool post body and having a manually operable power end and a weight end which is coupled to the carrier block such that, when the power end is actuated, the carrier block is moved from a normal position to a lifted position so as to quickly displace a tool held thereby away from a workpiece.