A curtain wall mullion anchoring system that does not require adjustability among the anchoring system components and the mullion to account for construction tolerance. The anchoring device can be accurately installed on a cured concrete floor slab at the proper in-and-out and left-and-right positions. A mullion connector is engaged with the anchoring device and the mullion to transfer reaction forces on the mullion to the building structure via the anchoring device. The mullion connector is slidably engaged with the mullion such that the mullion connector can slide along the length of the mullion and will be placed at the proper up-and-down position by simply sliding the mullion connector down the mullion to the anchoring device on the concrete floor slab.