Although at the start of the World War he preferred neutrality, after Italy and the Triple Entente became allies he was the main Italian negotiator of the Treaty of London (1915) which promised Italy substantial territorial gains in case of victory against Germany and Austria.
However, in November 1920, due to strong disagreements with the new Italian foreign minister, Count Carlo Sforza, Imperiali was removed from his post in London, replaced by Baron Giacomo de Martino and mostly excluded from post-war negotiations.