Geoffrey Mainwaring was born in Adelaide, South Australia, in 1912. He studied at the South Australian School of Arts and Crafts before becoming an art teacher at Thebarton Technical School (1928-36). He joined the 2nd Australian Imperial Force in June 1941 and after five months was transferred to the Engineers and employed as a Sergeant-Instructor, specialising in demolitions. In late 1942, he was sent to New Guinea as an Australian army artist on probation.

While in New Guinea Mainwaring battled with the climate and conditions: he had trouble receiving sufficient art supplies, oil paint would not dry, and there were countless problems sending completed works back to the Military History Section (MHS) headquarters in Melbourne. He returned to Australia for several months before being sent back to New Guinea as an official war artist. In February 1945, Mainwaring wrote to John Treloar, the officer in charge of the MHS:


I have been out on patrols with various platoons, and gained an understanding of the many trials and tribulations of the jungle fighter & For all the mental tensions, and somewhat nerve-wracking experience of the soldier in the depth of the foul and eerie jungle, there is little that is really spectacular, and combat in the jungle is so concealed, that it is difficult to get a picture of clashes with the enemy.

In March 1945, while attached to MHS (6th Division), Mainwaring completed two pictures specifically for inclusion in the 1945 Army Christmas book. By May he was back in Melbourne working on the book, before going on leave.

Mainwaring continued to work on paintings of the 6th Division fighting in the Wewak area of New Guinea. He was again in New Guinea when it seemed that the end of the war was imminent and was able to document the surrender and its aftermath on Bougainville, Nauru and Ocean Island. By May 1946 he was aboard HMAS Shropshire on his way to England to record victory parade scenes.

On 27 May 1947, Mainwaring was discharged from the Army, having served for five and a half years. He was appointed as an artist on a civilian basis until his paintings were completed in March 1948. In 1949 he was appointed Head of the Art School at the Ballarat School of Mines (now the University of Ballarat). He continued to paint commissioned portraits for the Australian War Memorial until the late 1950s. Mainwaring died in Ballarat in April 2000.

Mainwaring was adept at depicting a variety of subjects, from landscapes to portraits, and was very competent using a variety of media, including pencil, oils, and watercolour. He was a confident draftsperson, and completed many sensitively rendered portraits that give an insight into the character of the sitter. The Memorial holds 392 of his works of art, including many portraits, detailed drawings of military technology (tanks, aircraft and weaponry), and pen and wash studies completed in the field.