Atik Mustafa Pașa Mosque was a Byzantine church located in the region of Blachernai, in the northwestern corner of the city, near the land walls and the walls of the Golden Horn. It is approximately 100-150 meters east of the Church of the Virgin Mary of Blachernai.
While its date and identity are uncertain, several suggestions have been made including the churches of Sts. Peter and Mark, St. Thecla and St. Elias of Petrion. It displays many archaic elements suggesting it dates to the second half of the 9th century. If such a date could be sustained, it could be the earliest surviving post-iconoclastic church in Constantinople and the first of the cross-in-square type. It might have had an influence on other churches as well. It has been argued that this building type, with its heavy crossing piers and simple, barrel-vaulted corner bays, became "canonical" all across Russia in the course of the eleventh century. However it has also been argued this church dates to the eleventh or twelfth century.
The church was a cross-in-square church with three apses. It has been significantly altered over the course of history. Originally the proportions of the building must have seemed a lot lighter than today. This is due to the change in ground and floor level; in the Byzantine period, the floor was 1.50 meters lower than today. Furthermore, the drum of the Byzantine dome was taller and filled with windows.  Its current dome, which is low and windowless, dates to the Ottoman Era. In addition, a porch with the minaret replaced its narthex. Its exterior surfaces, especially its windows, have been thoroughly reworked, while interior has been plastered. In recent decades, undocumented restorations destroyed much evidence, making it difficult to date or identify the church. 
The church was converted into a mosque by Koca Mustafa Pasha (who also converted the Monastery of St. Andrew in Krisei) before his death in 1512. In the south-eastern corner bay of the mosque is the tomb of Jabir ibn Abdullah, a legendary warrior of Islam and companion of the Prophet Mohammed. The mosque is also known as Hazreti Cabir Mosque and has subsequently assumed further religious significance. The building suffered extensive damage from a fire in 1729 and an earthquake in 1894. It was restored in 1906 and in 1922.

In 1957, the American Byzantine Institute found frescoes of the Archangel Michael flanked by saints Cosmas and Damian, which date to the first half of the 15th century. They are no longer visible as they were later covered up during restoration. It is possible that the frescoes were part of a funerary chapel attached to the church.