Kasım Ağa Mosque is situated in the northwestern part of Constantinople, a short distance from the Monastery of Chora and the Cistern of Aetius. It is possible that Kasım Ağa Mosque, along with the nearby Odalar Mosque and the Ipek Cistern, once formed part of a monastery. 
Its date and identity are unknown. Due to its masonry, it has been suggested that it was a Palaiologan structure. It has no apse and was perhaps was an annex building for a monastery in the area.  
Although the district remained Christian for about two hundred years after the conquest, the building was converted by Kasım Ağa into a mosque in 1506. He also founded a waqf for the mosque. It was damaged by an earthquake in 1894 and then by a fire in 1919. It was restored in 1977, and the present minaret was erected in 1989.