He was born in Edinburgh, and educated at Edinburgh University and Gottingen University. After studying law in London, he was called to the bar in 1879, and was a rather successful lawyer. A few years before, he had been elected a Liberal member of Parliament for the Scottish seat at East Lothian. In 1895, he helped found the London School of Economics. After lecturing for a few years, in 1905, he was appointed Secretary of War under Henry Campbell-Bannerman's administration. In this post, he was responsible for several important British Army reforms, and was raised to a peerage in 1911, becoming the Viscount of Cloan. After having been appointed Lord Chancellor in 1912 by Herbert Asquith, he was forced to resign in 1915, after being accused of pro-German sympathies. After joining the Labour Party in 1918, he held the same position (of Lord Chancellor) under Ramsay MacDonald's Labour administration.
His brother was respiratory physiologist John Scott Haldane, and his sister was author Elizabeth Haldane. His only "famous" literary work was The Reign of Relativity (1921), which dealt with the philosophical implications of the theory of relativity.