Al- Mahdi (775-785) was the third Abbasid caliph. He helped with the decline of the empire. He had made efforts, however they obviously were not successful. After Al-Mahdi died, his older son, who took his place, died within a year by being poisioned. The next runner up for the throne was Harun Al-Rashid.

The powerful Barmakid family, which had advised the Caliphs before, gained even greater powers under al-Mahdi's rule, and worked closely with the caliph to ensure the prosperity of the Abbasid state.
The cosmopolitan city of Baghdad prospered during al-Mahdi's reign. The city attracted immigrants from all of Arabia, Iraq, Syria, Persia, and lands as far away as India and Spain.
Baghdad was home to Christians, Jews, Hindus, and Zoroastrians, in addition to the growing Muslim population. It became the world's largest city.
Al-Mahdi continued to expand the Abbasid administration, creating new diwans, or departments, for the army, the chancery, and taxation. Qadis or judges were appointed, and laws against non-Arabs were dropped.