Mattarella, 73, was elected by lawmakers on Saturday to succeed Giorgio Napolitano, who stood down two years into his second term citing fatigue at the age of 89.
Mattarella becomes Italy's 12th president since the country became a Republic after World War II.
His election has been seen as a political coup for Italy's Prime Minister Matteo Renzi.
The youthful, centre-left prime minister had backed Mattarella despite opposition from opposition leader and former premier Silvio Berlusconi.
Renzi's candidate comfortably carried the vote with 665 of the votes cast in a 1,009-member electoral college.
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He entered politics after his elder brother, who was president of the region of Sicily, was murdered by the Mafia in 1980.
Mattarella is also seen as a foe of Berlusconi, having once resigned from government over a media law he and other ministers regarded as overly favourable to the tycoon's television interests.
The new president is now being a member of Renzi's Democratic Party (PD) having started his career as a Christian Democrat.