Mariano Agate was a member of the Mafia. He was the boss of Mazara del Vallo Mafia family since the 1970s when he replaced the old boss Mariano Licari. He also was the boss of the mandamento of Mazara, including the Mafia families of Marsala, Salemi and Vita. Agate was a member of "Iside", one of the most powerful local Masonic lodges.
In the 1980s Agate supported the Corleonesi faction during the Second Mafia War and became the most important ally of Totò Riina in the province of Trapani. He also became a member of the Sicilian Mafia Commission.
Agate was arrested in 1982 for heroin trafficking. In 1985 he was sentenced to life for several murders, including those of judge Giacomo Ciaccio Montalto and the mayor of Castelvetrano, Vito Lipari. Agate has arrested again in 1992, after his release the year before. He has been sentenced to life for the murders of the Antimafia judges Giovanni Falcone and Paolo Borsellino.
Despite being jailed, in 2004 he was accused of running an international cocaine trafficking ring with several 'Ndrangheta clans (the Marando, Trimboli and Barbaro clans from Platì in particular), supported by his son Epifanio Agate and Salvatore Miceli, a fugitive in Colombia. Although incarcerated under the strict 41-bis prison regime he instructed his son on how to run the business.
Agate died at home on April 3, 2013. He had been granted house arrest for serious health reasons. Agate was ill for some time with cancer. The authorities banned a public funeral to guarantee public order and security.