This mosque was built on a rectangular plan measuring 75 m x 55 m. It is flanked by two round towers which served to catch the water running from the terraces. It seems the mosque did not have a minaret. The call to prayer was probably proclaimed from one of the towers. In front of the main entrance stands a porch claimed to be the first to figure in Maghrebin architecture. Harmoniously proportioned, this porch is roofed with a round horseshoe arch with mihrab-shaped niches placed high up on either side. Lower down, the different faces of the porch are ornamented with flat-backed niches. This decoration, a main feature of Fatimid monuments (best exemplified by the lateral façade of the Great mosque at Sfax) was carried over into the early Zirid period. This porch is reminiscent of the triumphal arches built by the Romans. This entrance porch leads into a courtyard surrounded by four galleries with a room at each corner. These are supported by arches and pillars rising from the courtyard on each side. Pointed arches for the north gallery simply rest on pillars whilst the three other galleries have round horseshoe arches and columns with capitals. From the southeast gallery one comes into the prayer-hall which has nine naves and three bays. The central nave and the transept form a T-shape. At their intersection is a dome supported on multifoil columns, under which a reproduction of the AH 5th-/ AD 11th-century mihrab has been built. This comprises a niche with fluted walls, each flute terminating in a shell motif. A cul-de-four on top of the niche is bordered by a horseshoe arch positioned on small corner columns incorporating Zirid capitals. The Great Mosque at Mahdiyya
was the inspiration for several Egyptian mosques, notably al-Hakim and al-Aqmar
mosques in Cairo.