He graduated 10th in the order of merit, and received a commission as a Subaltern in the Seaforth Highlanders in 1927. In 1929 he changed his name to "Baillie-Stewart" to make it sound higher up in the British class structure, under the belief that he was looked down upon by more senior officers, even though his father had been a Colonel and his mother was from a family with a long tradition of military service. He soon grew to dislike army life.
The court was told that Baillie-Stewart's treason had begun in 1931 when he met and fell in love with a German woman while holidaying in Germany, and decided to become a German citizen, writing a letter to the German Consul in London offering his services. Receiving no answer, he travelled to Berlin without permission to take leave, where telephoned the German Foreign Ministry and demanded to talk to an English speaker. This resulted in him making contact with a Major Mueller under the arches of the Brandenburg Gate, where he agreed to spy for Germany.
Using the pretext that he was studying for Staff College examinations, he borrowed from the Aldershot Military Library specifications and photographs of an experimental tank (the Vickers A1E1 Independent) and a new automatic rifle, and notes on the organization of tank and armoured car units. It was charged that he had sold this material to a German known as "Otto Waldemar Obst", in return for which he received two letters signed "Marie-Luise," one containing ten £5 notes, and the other four £10 notes. Evidence was also produced that he had also made several trips to Holland to meet with his handlers. MI5's files have since shown that Marie-Luise had been merely a figment of his controller's imagination; Major Mueller's covername was Obst (fruit) and Baillie-Stewart's was Poiret (little pear), while Marie-Luise, a type of pear, was used to conceal their correspondence.
He was imprisoned for five years, which he served at the Tower of London, the last British citizen to be imprisoned there.
Following the Anschluss of 1938, Baillie-Stewart was able to return to Austria, where he made a small living from operating a trading company. He applied for naturalization but the application was delayed by bureaucracy at the Ministry and he did not become a German citizen until 1940.
In July 1939, Baillie-Stewart attended a friend's party where he happened to hear some German English language propaganda broadcasts. He criticized the broadcasts, and was overheard by a guest at the party who happened to work at the Austrian radio station. He informed his superiors of Baillie-Stewart's comments, and after a successful voice test in Berlin, Baillie-Stewart was ordered by the German Propaganda Ministry to report to the Reichsrundfunk in Berlin, where he became a propaganda broadcaster. Baillie-Stewart made his first broadcast on the "Germany Calling" English language service a week before the United Kingdom declared war on Germany, reading Nazi-biased "news".
It was Baillie-Stewart who made the broadcast which led the pseudonymous Daily Express radio critic Jonah Barrington to coin the term "Haw-Haw". The nickname referenced Baillie-Stewart's exaggeratedly aristocratic way of speaking. When William Joyce later became the most prominent Nazi propaganda broadcaster, Barrington appended the title and named Joyce "Lord Haw-Haw", since the true identity of the broadcaster was unknown at the time.
By the end of September 1939 it was clear to the radio authorities that Joyce, originally Baillie-Stewart's backup man, was more effective. Baillie-Stewart, who had gradually became disenchanted with the material that he had to broadcast, was dismissed in December 1939 shortly after his last radio broadcast. He continued to work in Berlin as a translator for the German Foreign Ministry, and lectured in English at Berlin University. In early 1940, he acquired German citizenship.
In early 1942, Baillie-Stewart made a brief return to radio under the alias of "Lancer", making several broadcasts for both the Reichsrundfunk and Radio Luxembourg. He spent much time avoiding the more blatant propaganda material he was asked to present.
In 1944, Baillie-Stewart had himself sent to Vienna for medical treatment, where he was arrested in 1945 in Altaussee, while wearing "chamois leather shorts, embroidered braces and a forester's jacket" and was sent to Britain to face charges of high treason.
Baillie-Stewart pleaded guilty, and was sentenced to five years' imprisonment following which he moved to Ireland under the pseudonym of James Scott, married, and had two children before dying in a Dublin bar in 1966.