He also served as leader of the Union of Democrats for Europe (UDEUR), a minor centrist and Christian-democratic Italian party.

During the same period, he was a member of the Senate of the Republic and determined the narrow majority of Romano Prodi's government, which ended when he started the 2008 Italian government crisis that led to Prodi's resignation as prime minister and Silvio Berlusconi return to power after the snap election that ensued.

His career as a journalist and his beginnings in political life have been widely described by himself in various interviews, cited for example in the book La casta by Sergio Rizzo and Gian Antonio Stella, where his hiring at the RAI, Italy's public broadcasting, had been helped by a recommendation from the DC politician Ciriaco De Mita.

[3] In 1998, after the fall of the first Prodi government, Mastella decided to follow Francesco Cossiga, lifetime senator and former Italian president.

This new political party, which supported the new centre-left coalition government led by Massimo D'Alema, lasted one year.

Mastella and the then Sicily president Salvatore Cuffaro were subjects of a scandal when it was revealed that they had been the best men of Francesco Campanella, a former member of the Sicilian Mafia who helped the boss Bernardo Provenzano when he was a fugitive from the law.

As Minister of Justice, Mastella received an advice of judicial proceedings in February 2007 from the Naples prosecutors' office.

Mastella's wife, Sandra Lonardo, at the time also a UDEUR politician who was the acting president of the Regional Council of Campania.

[11][12] Mastella's decision occurred a few days after the Constitutional Court of Italy confirmed that there would be a referendum to modify the electoral system.

[14][15] The fall of the government disrupted a pending election-law referendum that, if it had been passed, would have made it harder for small parties like Mastella's to gain seats in the Italian Parliament.

[23] On 3 June 2020, he left FI to create a regional list named Us Campanians (NC) in support of centre-left coalition incumbent president Vincenzo De Luca for his re-election campaign.

[3] In 1975, Mastella married Sandra Lonardo, a native of Benevento whom he met during a visit to an uncle in Oyster Bay, New York, where she spent a good part of her youth.