[10] Sinclair served on the Western Front during World War I, in 1915 as aide de camp to J. E. B. Seely who commanded the Canadian Cavalry Brigade.

[13] It was Sinclair who introduced the prominent British agent Sidney Reilly to Nikolai Alekseyev, intelligence chief of the White Russian leader Alexander Guchkov.

[8][16] In July 1931, a meeting took place at Sinclair's house, where Oswald Mosley and Harold Nicolson met Churchill, Lloyd George and Brendan Bracken, to discuss a political alliance.

[17][18] About a month later, the Liberal Party joined the National Government of Ramsay MacDonald, with Sinclair appointed Secretary of State for Scotland.

[20] In 1932, he, together with other Liberal ministers led by Herbert Samuel, resigned from the government in protest at the Ottawa Conference introducing Imperial Preference.

During the Abdication Crisis of 1936, his name was put forward as a possible leader of a government that might be formed if Edward VIII held onto the throne against the wishes of the Baldwin administration.

[16] He joined the Anti-Nazi Council of Eugen Spier, with Churchill and Violet Bonham Carter, Margaret Bondfield and Hugh Dalton.

[24] Public opinion at this point of the later 1930s by no means agreed, and John Alfred Spender attacked Sinclair in The Times on foreign policy, claiming that he, like the League of Nations Union, wished for war with the Axis Powers.

[26] During parliamentary debate over the Munich Agreement he attacked Prime Minister Neville Chamberlain for "wilting" to Nazi Germany and tossing "justice and respect for treaties... to the winds.

During the May 1940 British war cabinet crisis after the fall of France he sided with Churchill against Lord Halifax's plan to seek a negotiated settlement with Nazi Germany mediated by Fascist Italy.

Towards the end of the war, he found himself at odds with Churchill, arguing against Bomber Harris' strategy for the Bombing of Dresden and other German cities.

They had four children: The Southern Railway named a Battle of Britain Class Light Pacific steam locomotive "Sir Archibald Sinclair".