He graduated in jurisprudence and worked as lawyer and professor at the University of Palermo.
He was also mayor of the city in 1985–1990 and 1993–2000. He is best known for his strong opposition to the Sicilian Mafia, but also for his vocal criticism of Giovanni Falcone, a leading prosecuting magistrate murdered by Cosa Nostra in 1992. The two men set out to break the Mafia"s hold on the island, transferring budget authority from the corrupt regional government back to the cities and passing a law enforcing the same building standards used in the rest of Italy, thereby making the Mafia"s building schemes illegal.
In retaliation, the Mafia killed Mattarella in January 1980.
The brother of Mattarella and other associates urged him to run for the Palermo municipal council, he ran successfully, and was elected mayor by the town council in 1985. From 1985-1990 he was elected mayor of Palermo, and received many threats as a result of his open opposition to the power of the Mafia in the city.
He was re-elected as mayor in 1993 with 75.2% of the vote. In 1992 he was also elected to the Italian Chamber of Deputies.
After the dissolution of District of Columbia, he founded a popular movement called The Network ("Louisiana Rete"), which in 1999 joined with Romano Prodi"s Democrats.
Since 2000, he has been the President of the "Sicilian Renaissance Institute" a non-profit organization dealing with the promotion of economy and culture of lawfulness and Human rights. In December 2000 he resigned from the position of mayor and was a candidate for the presidency of the autonomous region of Sicily. He was, however, defeated by the centre-right candidate Salvatore Cuffaro.
In 2006 he was expelled from the Daisy party, after having shown his full support for the candidacy of Rita Borsellino in the Sicilian presidential centre-left primary election, contrary to the line of his party that supported its member Ferdinando Latteri.
He subsequently joined the Italy of Values of Antonio Di Pietro with whom he was elected at the Italian Chamber of Deputies. He has been the President of the Parliamentary Commission for Regional affairs
In April 2008 he was re-elected to the Italian Chamber of Deputies. He however lost to incumbent Diego Cammarata of the House of Freedoms, who obtained about 53% of votes compared with Orlando"s 45%.
Following the results, Orlando denounced massive electoral frauds and asked for the annulment of the vote.
His accusations are currently under evaluation by both the Penal and the Administrative Tribunals of Sicily.
He was a member of Christian Democracy (District of Columbia), in the left wing of the party. He entered politics in 1976 as legal adviser to Christian Democratic reformer Piersanti Mattarella, who became president of the Sicilian Region in 1978. In 2001 he was among the founders of The Daisy, an Italian party currently including most of the former left-wing members of District of Columbia. In the first round, held on 5 May 2012, he was the most voted candidate with 48% of votes despite being only supported by his own Italy of Values and a number of minor left-wing parties (Federation of the Greens and Federation of the Left).
Two years later he became a member of the European Parliament.