Ainslie was born in London, England in 1911 to Harold Roberts and Rose (née Dougall). His early education was at St James's School, Clapton. The family migrated to Australia in 1922, staying first at Ardrossan before settling in Adelaide. Ainslie resumed his schooling at Westbourne Park Primary School, Blackwood in 1923 and was school dux and first in the state of South Australia in his Qualifying Certificate in 1926. His paintings and drawings from this period demonstrate proficient drafting skills and adept use of colour, along with affection for the Australian landscape and ships, locomotives, buildings and bridges as favourite subjects.
He married Melva Jean ('Judy') Andrewartha on 27 February 1937. Ainslie was a keen photographer, and was for some time president of the Adelaide Camera Club. Small in stature, but fit through swimming and working out in a health studio, he was rejected from military service during World War II because of a history of rheumatic fever. But joined the Volunteer Defence Corps, where his experiences inspired some fine cartoons. Ainslie and Judy Roberts' son Rhys was born in 1944.
In 1952, he met Charles Pearcy Mountford, who, like Ainslie, was a keen photographer. Mountford was also a largely self-taught ethnologist, writer and documentary film maker who, though he would take a Diploma of Anthropology from Cambridge in the late 1950s, worked and remained largely outside academic circles.
For several years, the two took journeys around South Australia to photograph caves and rock carvings, but in 1956, they made the first of several trips to the Centre. Mountford collected myths and legends from tribal people, and Ainslie sketched and painted people and places. They made friends with characters like Bill Harney, a bushman, raconteur and writer, and Gwoya Jungarai or "One Pound Jimmy", famous for being depicted on earlier Australian stamps and in Walkabout magazine. With Mountford, he formed a company that produced the first tourist guides to Ayers Rock and The Olgas.
The exhibition was a sellout, and early in 1964, the poet Ian Mudie, who was publishing manager of Rigby, proposed a book of the works. Ainslie's format was simple – one myth to an opening, a painting on one side and the text and a line drawing on the other. The Dreamtime was first published in 1965 and has been reprinted many times.Wikipedia