An apparatus and a method for manufacturing a mask assembly and a method of manufacturing a display device are provided. The apparatus may include: a tensioning part configured to be spaced apart from a mask frame comprising at least one opening, the tensioning part configured to tension a mask sheet in at least one of a first direction and a second direction, the mask sheet comprising a cell area corresponding to the at least one opening, and a dummy portion arranged outside the cell area; a pressing part configured to correspond to the dummy portion and press the dummy portion in a third direction intersecting a plane in which the first direction and the second direction extend; and a header configured to irradiate a laser beam toward the mask sheet to fix the mask sheet to the mask frame.