Abstract: A mullion assembly mountable in a door frame includes a bottom fitting attached to a floor and a top fitting attached to the door frame above the bottom fitting. The top fitting supports a latch engagement assembly for engaging a latch assembly positioned in a mullion cavity of a mullion. The latch assembly has a latch extendable from the mullion cavity to engage the latch engagement assembly supported by the top fitting and hold the mullion in a fixed position. Retraction of the latch is controlled by a key lock assembly connected to the latch assembly through an actuating link assembly. The key lock assembly permits key controlled disengagement of the latch from the latch engagement assembly, followed by temporary removal of the mullion from its position in the door frame.