Sonnino was born in Pisa. In 1878, he started a newspaper (La Rassegna Settimanale), which changed from weekly economical reviews to daily political issues. In 1893, after working in several governmental positions, he became finance minister of Italy. He worked in the opposition after the fall of his party from power as a result of the lost Battle of Adwa.
After the events in 1914, Sonnino was initially supportive to the German and Austria-Hungary side, but after becoming foreign minister in November 1914, he sided with the Allied forces, and signed the Treaty of London in 1915. Italy consequently declared war on Austria-Hungary on May 23, 1915.
When his territorial ambitions towards Austria-Hungary were shattered during the Paris Peace Conference, his party lost power again, and Sonnino retired from politics.