He graduated in law at the University of Naples in 1959.
After becoming police vice-commissar, he worked in Brescia and then in Forlì. After one year in Rome as a functionary of the Ministry of Treasury, he become a magistrate in 1964. During his career as prosecutor, Imposimato was in charge of investigation for the kidnapping of Aldo Moro, Mehmet Ali Ağca"s attempted assassination of Pope John Paul II, the assassination of banker Michele Sindona, and several Mafia trials.
In 1981 he was in charge of the trial against the Banda della Magliana.
He also dealt with violations of human rights in South America. In 1987 Imposimato was elected to the Italian Senate.
In 1992 he was re-elected to the Chamber of Deputies. He believes that the Federal Bureau of Investigation was aware of the presence of the 9/11 terrorists in the United States but did not alert the Central Intelligence Agency. In January 2015 he was nominated by the Five Star Movement as its candidate for the 2015 presidential election after an online survey by supporters of the movement, winning 32% of the votes.
He was a member of the Parliamentary Anti-Mafia Commission in three consecutive legislatures.