The swearing-in ceremony took place in front of a joint-parliament assembly comprising 1,009 lawmakers and regional representatives who elected the new president with 665 votes on Jan 31, 2015, Xinhua reported.
Mattarella, a former professor of parliamentary law and lawyer, resigned as constitutional judge, a post he held since 2011.
The 73-year-old new president succeeded Giorgio Napolitano, 89, who quit last month due to his old age.
Born on island of Sicily into a Catholic family, Mattarella entered politics in the 1980s after his brother was killed by the mafia.
Mattarella’s candidacy was promoted by Prime Minister Matteo Renzi, who said his election will give a “turbo” boost to his centre-left government’s ambitious plan of structural reforms.