His son and namesake is accused of organising the massacre on a remote road to stop Mangudadatu challenging him for the governorship in Monday's national elections.
'Victory for victims'
"This is a victory for the victims of the massacre and for the people of Maguindanao who have been suffering under oppressive rule," Mangudadatu told a crowd of supporters gathered at the capital building in Shariff Aguak town.
"But the battle is not over yet. The culprits have to pay first. Our battle against poverty has also just started."
Escorted by heavily-armed soldiers and combat-trained police officers, Mangudadatu rode in an armoured vehicle to the local election agency office to be proclaimed winner.
The convoy passed through the town where his wife and relatives were murdered along with 30 journalists and 20 other civilians last November.
They had been on their way to witness the filing of nomination papers for Mangudadatu to stand against Andal Ampatuan Jr in the elections.
Political violence is common in the southern island of Mindanao, which includes Maguindanao. Muslim and Communist insurgencies have simmered there for decades.
But in a country that has long grown accustomed to election-related advice, the
killings last year were the worst in the country's history.
Ampatuan Sr, his four sons and a brother are being held in a maximum detention facility in Manila, awaiting trial on 57 murder charges.
Two family members nearly escaped prosecution after the justice department dropped them from the list of accused last month, but the decision was reversed on May 5.