After finishing his education he was admitted to the Bar (permitted to practice as a barrister), which he did for a decade before becoming involved in politics in 1906 when he entered the House of Commons (the elected house in the British system of government rather than the hereditary House of Lords).
During World War I, he served in a number of junior ministerial positions in the government, including several devoted to the war effort. It was perhaps this that led to a memorandum outlining his ideas for the avoidance of war, which according to Cecil was the "first document from which sprang British official advocacy of the League of Nations".
Through the early 1920's Cecil worked as an official British delegate to the league in a variety of capacities, but eventually he tired of his Cabinet colleague's indifference to it and retired from his political offices and, by 1932, from any official British role in the League, instead acting as an external advocate for it.
He attended the final meeting of the League in 1946 as it was superseded by the United Nations, a development that Cecil was happy to see.
He married Lady Elanor Lampton.