A standing seam panel system includes an internal structural mullion. A pair of panels meet at a joint and have standing flanges and detents. The mullion includes a main extrusion and a secondary extrusion, the main extrusion having an innermost portion, and a main hooked portion opposite the innermost portion. A secondary extrusion is attached to the main extrusion and has a leg hooked portion opposite the main hooked portion to form a pair of hooked portions, the hooked portions engaging the detents of the panels. Slotted holes are defined within said main extrusion, and screw fasteners pass through the slotted holes of the main extrusion and into a building substrate, wherein the innermost portion of the main extrusion is attached to a building substrate such that the mullion is adapted to move vertically along the building substrate to accommodate expansion or contraction of the panels.