During the war of 1848 he interrupted his studies to serve as a volunteer against Austria, and was wounded at the battle of Rivoli. On the conclusion of peace he entered the Piedmontese foreign office; he accompanied Victor Emmanuel and Cavour to Paris and London in 1855, and in the following year he took part in the conference of Paris by which the Crimean War was brought to an end.
After the meeting at Plombières between Cavour and Napoleon III Nigra was sent to Paris again to popularize a Franco-Piedmontese alliance, Nigra being, as Cavour said, the only person perhaps who knows all my thoughts, even the most secret. He was instrumental in negotiating the marriage between Victor Emmanuel's daughter Clothilde and Napoleon's nephew, and during the war of 1859 he was always with the emperor. He was recalled from Paris when the occupation of the Marche and Umbria by the Piedmontese caused a breach in Franco-Italian relations, and was appointed secretary of state to the prince of Carignano, viceroy of the Neapolitan provinces.
When Napoleon recognized the kingdom of Italy in 1861, Nigra returned to France as minister-resident, and for many years played a most important part in political affairs. In 1876 he was transferred to St Petersburg with the rank of ambassador, in 1882 to London, and in 1885 to Vienna. In 1899 he represented Italy at the first Hague Peace Conference. In 1904 he retired, and he died at Rapallo on 1 July 1907. He was created count in 1882 and senator in 1890.
Nigra was a sound classical scholar, and published translations of many Greek and Latin poems with valuable comments; he was also a poet and the author of several works of folk-lore and popular poetry, of which the most important is his Canti popolari del Piemonte.