Adrian was born in Wolverhampton in 1888, the son of Walter Henry Trimnell and Alice S. D. Winter. In 1901, they were living at 9 Compton Road, together with his sister Katherine and brother Geoffrey. Adrian attended Wolverhampton Grammar School and then Oaklands, Rugby. At some point, Adrian held a commission in the Royal Field Artillery. He became an engineer, but in 1911 he moved to New Zealand to take up farming. In 1913 he started managing a sawmill.
When war broke out, he and 180 of his pals marched into Wellington and all enlisted together, serving with the Wellington Regiment of the New Zealand Expeditionary Force (number 10/541). He soon received promotion to sergeant and later Lance Corporal, and served in the trenches during the Turkish raid on Egypt. An article appeared in the Midland Counties Express on 19 June 1915 stating that his parents had received official intimation that he had been killed in Turkey, on 29 April 1915. The article gave a lot of details about Adrian’s life, stating that he was “a good shot and rider,…and especially expert was he with the axe, and it was said that no one for miles around could fell trees with so much facility.” The article also told of his exploits whilst at Wolverhampton Grammar School:
As a boy he evinced the same spirit of adventure. On one occasion, along with a few other Grammar School boys, he made hundreds of snowballs, and, standing on the roof of his father’s fine old house, held up the traffic in Compton-road. The Chief Constable was one of those who were pelted, and had to wait until the lads wanted their tea before he could get past.