Self-educated, he studied law.
He began practice in 1898. He was elected as a Socialist to the Chamber of Deputies from Ostiglia. In September 1902, when the Italian Socialist party split into Reformist and Revolutionary factions, Bonomi became one of the leaders of the Reformist faction. In 1916 he became minister of labor under Paolo Boselli, after having been expelled from the Socialist party (1912) for advocating the entry of Italy into the anticipated World War I on the Allied side. In 1919 he was a minister in the Orlando cabinet and later served briefly in the Francesco Nitti cabinet. He entered the Giovanni Giolitti cabinet in 1920 as a minister of war and later was minister of finance. In 1921, after the fall of the Giolitti government, Bonomi became premier of Italy; however, in February 1922, because of Liberal-Popolari (Catholic Popular) dissension, the government fell. Bonomi denounced Fascism in vigorous terms and was sharply critical of the Socialist party and its tactics during the period in which Mussolini and his Black Shirts rose to power. In 1924 Bonomi delivered his last political speech, denouncing Fascism, and afterward retired from public life.
Although he did not leave Italy he was unable to carry on his law practice. He spent the next 20 years in straitened circumstances, his only income derived from a small pension and payments for help extended to other lawyers in preparing cases. Finally, in June 1944, Bonomi returned to power in place of Marshal Pietro Badoglio as the head of the new liberation government in Italy, with a nonfascist cabinet. The following November 26 he retired because of political disunity, but again became premier in December 1944, and served until June 1945. In April 1948 he was elected president of the Italian Senate as a political independent, and was chairman of the Italian delegation to Great Britain in July 1948, which sought the return of Italian colonies to Italian rule. During World War II Bonomi was chairman of the Italian underground liberation committee. He wrote Mazzini triumviro della Republica Romana (1937), and other works. He died in Rome on Apr. 20, 1951.