Bonomi was born in Mantua, Italy. He was elected to the Italian Chamber of Deputies in 1909, representing Mantua as a member of the Italian Socialist Party. He was among those expelled from the party in 1912, for his advocacy of reformism and moderation, as well as his support for the Italian invasion of Libya. Bonomi joined the Italian Reformist Socialist Party, and supported Italy's participation in World War I on the side of the Triple Entente.