Riddell was born in Brixton Heath, London, the son of James Riddell, a photographer, and Isabel (née Young). His education was funded by one of his mother's five sisters and her husband. He became a clerk in a solicitor's office, and qualified as a solicitor himself in 1888, being placed first in all of England in his final exams.
Riddell later abandoned the law, however, and went into the newspaper business. By 1903 he was managing director of the News of the World and also owned other newspapers. A close friend and ally of David Lloyd George, he was knighted in 1909, on the recommendation of H. H. Asquith. During the First World War, he liaised between the government and the press and represented the British press barons at the Paris Peace Conference and later peace conferences. For these services he was created a Baronet, of Walton Heath in the County of Surrey, in 1918 and raised to the peerage as Baron Riddell, of Walton Heath in the County of Surrey, in the 1920 New Year Honours. He was the author of several books, among them Some Things that Matter (1922),Lord Riddell's War Diary, 1914–18, and Lord Riddell's Intimate Diary of the Peace Conference and After. He was not impressed by his contemporary, Winston Churchill.