Rupert Henry Treganowan was born on 14 January 1896 at Clifton Hill, Victoria. As a 21-year-old he enlisted with the Australian Imperial Force (AIF) at Bendigo on 12 January 1917, where he was given the service number 18321 and the rank of sapper. A telegraphist before the war, Treganowan was earmarked for a signals unit, and on 9 May 1917 he left Sydney aboard the troopship HMAT Port Sydney.

Arriving in Egypt the next month, Treganowan was attached to a signal company for training. In September he joined the 1st Australian Airline Section of the Desert Mounted Corps whose role it was to run telegraph and telephone wires. Throughout 1917 and 1918 he detailed in letters home the conditions he and the Australian Light Horse faced, as well as the progress of the campaign in Palestine. In July 1918, Treganowan joined the 4th Signal Troop of the 2nd Signal Squadron and in the closing months of the war wrote about several of the major battles, including the Battle of Megiddo in September 1918. He was with the Australians when they entered Damascus at the conclusion of the war. With the surrender of the Ottoman Empire in October, the Australians were sent back to Egypt where they played a pivotal role in restoring order during the uprising of March and April 1918, something which Treganowan recorded in his letters. He left Egypt to return to Australia in July 1919.

Several years after returning to Australia, and on 25 August 1921, Treganowan married Alice Winidred Longmore. He passed away on 23 April 1952 at the Royal Melbourne Hospital and was buried three days later at Melbourne General Cemetery. His daughter Nancy compiled a transcript of his wartime letters in 1987.