Al-Mahdi was Sudan’s last democratically elected prime minister before he was toppled in 1989 by the military coup that brought former President Omar al-Bashir to power.
He died on Thursday, three weeks after being taken to hospital in the United Arab Emirates, al-Mahdi’s family said.
Sadiq al-Mahdi was Prime Minister of Sudan from 1966 to 1967 and again from 1986 to 1989.
He was head of the National Umma Party and Imam of the Ansar, a sufi order which pledges allegiance to Muhammad Ahmad.