[1] He was born to Antonio Segni, himself a prominent politician, and Laura Carta Caprino, who would have a total of four children.

[2][3] Prior to his political career Segni studied law at the University of Sassari, following in the footsteps of his father.

[4][5] Following his graduation, Segni moved to Padau where he worked under Luigi Carraro, a four-time Christian Democrat Senator, and taught at university.

[13] Riding the momentum from the 1991 Italian electoral law referendum, which he supported, Segni contributed to the establishment of the Democratic Alliance in 1992.

[18] In 1993, these reforms led to the Mattarellum law which he said would bring stability to Italy by making it more similar to Anglo-Saxon democracies.

[5][14] In March 1994, he founded the Segni Pact from the remains of the previous Democratic Alliance and Populars for Reform.

[citation needed] In 1999, Mario Segni returned to the domestic political scene, with the aim of attempting to abolish proportional quotas via referendums.

[6] During the 2006 election for the Presidency of the Italian Senate, Segni supported Franco Marini over former Christian Democrat head Giulio Andreotti, citing a respect for the old regime but a desire to see more progress in Italian politics and a fear of a return to the corruption prior to the Mani Pulite investigation.

This placed Segni alongside politicians such as Gianni Alemanno, Angelino Alfano, Mercedes Bresso, Riccardo Illy, Renato Brunetta, Antonio Martino, Giovanna Melandri, Arturo Parisi, Daniele Capezzone, Stefania Prestigiacomo, Gaetano Quagliariello, and Giorgio Tonini in the fight for a majority based electoral system.