Mattarella is Italy's 12th president and head of state since the country became a republic after the Second World War.
If that number sounds low, given the 65 governments that have ruled Italy in the same period, that's because this role changes only every seven years. This is to avoid the president being too closely tied to any one parliament; both houses have a five-year term, but in practice they tend to change more frequently.
Ordinarily, the president mostly acts as a figurehead: Mattarella regularly represents Italy on trips abroad or in national speeches on key dates. In between governments, his role becomes more central, including dissolving parliament, calling elections, supervising coalition talks, and encouraging parties to form a workable government.
Put simply, the job description is to oversee the running of the country, by ensuring the constitution is followed and Italy remains united. In his own inaugural speech, Mattarella described his role as that of a "political referee".
Mattarella is a well-respected figure in Italian politics, though he was relatively little known by the public and press before he was sworn in in 2015. The 75-year-old was born in the Sicilian capital of Palermo (he's the first Italian president from the region), and studied law in Rome.
After qualifying, he taught law in his hometown, with a focus on constitutional law and Sicily's regional government. His elder brother Piersanti was elected regional president of Sicily, and it was his brutal murder in 1980 at the hands of a local mafia group that persuaded the younger Mattarella to enter the political arena.
A gunman from the Cosa Nostra organized crime group shot at Piersanti as he got into his car to go to an epiphany mass on January 6th, 1980. Sergio was quick to arrive at the scene and comforted his brother, who died in his arms on the way to hospital.
Photo: Paolo Giandotti / Italian Presidency / AFP
Three years later, Mattarella was elected a member of Italy's Chamber of Deputies, the Lower House of parliament where he represented the Christian Democrats party. Over a long career, he served terms as Minister for Education, Deputy Prime Minister, and Minister for Defence. Among his key achievements were a decisive role in the abolition of obligatory military service, and the creation of an electoral law which bore his name and was used between 1994 and 2001.
In 2008, Mattarella chose not to run in parliamentary elections, and in 2011 was elected a Constitutional Judge. Four years later, he was elected president, and vowed to prioritize tackling corruption and organized crime while in office, as well as getting the country back on track after a crippling recession.
Friends and foes
Mattarella is a centre-left-leaning politcian who started his career as a member of the now-defunct Christian Democrats, which his father Bernardo helped create. Later he was one of the founders of the Democratic Party (PD) and that party, led at the time by Matteo Renzi, endorsed him in the 2015 presidential vote, which he won with a comfortable majority.
He is generally regarded as quiet and reserved; news agency Ansa described him as "almost mute", having made only 29 public statements in his time in parliament. But when Mattarella chooses to take a side, he makes his words count. He's been outspoken about several issues in the past, leading to clashes with some of the main players on the Italian political stage.
One of his main 'adversaries' has been Forza Italia leader and four-time Prime Minister Silvio Berlusconi. He resigned from government in 1990 over a media law which he and several other ministers argued was passed to support Berlusconi's personal interests by allowing the tycoon to expand his media network nationwide. Mattarella also described the admission of Forza Italia to the European People's Party as an "irrational nightmare".
League leader Matteo Salvini has also been critical of Mattarella, on his election describing him as "yet another 'catto-communista' (communist Catholic)", despite Salvini's own Catholic faith.
More surprisingly, the president has also clashed with singer Madonna. In 1990, as Minister for Education, he joined Italian Catholics in protesting against the star's 'Blonde Ambition' tour coming to Rome. The Vatican had labelled her music "blasphemous and deviant" while Mattarella said it was "an offence to good taste".