His intellectual distinction and political industry made him a valuable member of the Liberal party. As soon as the late 1860s, he acted as chairman of the royal commission on secondary education. In 1885 he was made Under-Secretary of State for Foreign Affairs but he had to leave office after the electoral defeat of Gladstone in the same year; in 1892 he joined the last cabinet of Gladstone as Chancellor of the Duchy of Lancaster, i.e. as Minister without distinct portfolio; in 1894 he was appointed President of the Board of Trade in the new cabinet of Lord Rosebery, but had to leave this office with that whole Liberal cabinet as soon as 1895. After a decade of parliamentary opposition, in Sir Henry Campbell-Bannerman's cabinet in 1905 he was made Chief Secretary for Ireland; but even this time his cabinet post was held only for a brief period, because as soon as February 1907 Bryce was appointed British Ambassador to the United States of America . He kept this diplomatic office until 1913) and was very efficient in strengthening the Anglo-American friendship. The German ambassador in Washington, Graf Heinrich von Bernstorff, later admitted how relieved he felt that Bryce was not his competitor for American sympathies during the World War period, when Bernstorff managed to secure the neutrality of the USA at least until 1917.
Bryce had a lot of American friends in politics and science. One of the most prominent was US President Theodore Roosevelt.
Meanwhile his academic honors from home and foreign universities multiplied, and he became a fellow of the Royal Society in 1894. In earlier life he was a notable mountain-climber, ascending Mount Ararat in 1876, and publishing a volume on Transcaucasia and Ararat in 1877; in 1899-1901 he was president of the Alpine Club. From his Caucasian journey he brought back a deep distrust of Ottoman rule in Asia Minor and a distinct sympathy for the Armenian people. In 1907 he was made a Member of the Order of Merit by King Edward VII, and after his retirement as ambassador and his return to Great Britain he was created Viscount Bryce of Dechmount in the County of Lanark in 1913. Thus he became a member of the House of Lords - that contested parliamentary body his own Liberal Party had bitterly fought the previous years, the powers of which had been curtailed in the Liberal Parliamentary Reform of 1911. He was President of the British Academy from 1913 to 1917.
Following the outbreak of the First World War, Lord Bryce was commissioned by Prime Minister Herbert Asquith to give the official Bryce Report on alleged German atrocities in Belgium. The report was published in 1915, and was damning of German behavior against civilians; Lord Bryce's accounts were confirmed by Vernon Lyman Kellogg, director of the American Commission for Relief in Belgium, who told the New York Times that the German military enslaved hundreds of thousands of Belgian workers, and abused and maimed many of them in the process.
Bryce also strongly condemned the Armenian Genocide that took place in the Ottoman Empire mainly in the year 1915. Bryce was the first to speak on that subject in the British parliament (House of Lords) in July 1915, and later - with the assistance of the historian Arnold J. Toynbee - he produced a documentary record of the massacres, published by the British government in 1916 as the Blue Book. In 1921, former Ambassador Bryce wrote that the Armenian genocide had also claimed half of the population of Assyrians in the Ottoman Empire, because "similar cruelties" were perpetrated upon them.
During the last years of his life, Bryce served at the International Court at The Hague, supported the establishment of the League of Nations, and published a book about Modern Democracy in 1921 with quite critical remarks about post-war mass democracy; e.g. he strongly opposed the new right to vote for women.
He died on January 22, 1922 in Sidmouth, Devon, on the last of his lifelong travels.