After leaving university, Rees wrote first for the "Manchester Guardian". In 1936, he became assistant editor of "The Spectator", for which he travelled to Germany, Russia, Spain, and Czechoslovakia. Though a Marxist during most of the 1930s, the Hitler-Stalin Pact turned him from communism and led him to enlist before the UK entered the war. During World War II, he joined the Royal Artillery and rose to second lieutenant in the Royal Welch Fusiliers. After the army, he resumed work at "The Spectator". In 1946, he then became an administrator for H. Pontifex & Son and may have started working for MI6. Rees's daughter confirms that he worked for MI6 then and until at least 1949: "...And in the afternoons he went to 54 Broadway, next door to St. James's Park tube station, the offices of SIS (or MI6), where he worked for the Political Section which... assessed and evaluated information..."