Castelli was born in Lecco on 12 July 1946.^[1] He holds a degree in mechanic engineering.^[1]

Castelli is an engineer, and has been in politics with the Northern League since 1986. In 1992, Castelli was elected to the Chamber of Deputies and was re-elected in 1994. From 1996 to 2013, he was a member of the Senate of the Republic, and for two years (1999–2001) was chairman of parliamentary group of the Northern League in the Senate.

In fall 2004, Castelli completed a highly controversial reform of the judiciary (Castelli Reform).^[2] Initially, the then Italian president Carlo Azeglio Ciampi, even though he had been pressured by Castelli to do so, refused to sign the bill. The law was passed again in spring 2005.^[3]

As a minister, Castelli refused to sign Ciampi's decision to pardon Adriano Sofri, leading to a conflict of powers in which Ciampi, as the president of the Italian Republic, tried to reassert its exclusive competence in the matter (as stated in article 87 of the Constitution of Italy), and Castelli claimed that, as justice minister, he had the right to object. He also refused to request the extradition of 22 alleged CIA agents from the United States who were implicated in the kidnapping and torture of Hassan Mustafa Osama Nasr. On 13 December 2012, Castelli said during the satirical talk show of Rai Radio 2 Un giorno da pecora that he would not seek re-election to the 2013 Italian general election.