After studying law in London, he was called to the bar by Lincoln's Inn, in 1879, and became a successful lawyer. He was taken on at 5 New Square Chambers by Lord Davey in 1882 as the junior. Haldane's practice was a specialism in conveyancing; a particular skill for pleadings at appeal and tribunal cases, bringing cases to the Privy Council and House of Lords. By 1890 he had become a Queen's Counsel. By 1905 he was earning £20,000 per annum at the Bar (). He became a bencher at Lincoln's Inn in 1893. Amongst his early friends was Edmund Gosse, the librarian of the House of Lords library.