Mary Cockburn Mercer was the third daughter of William Cockburn Mercer, an early settler in Springwood, Wannon, Western Victoria. Born in Scotland while her Australian mother was visiting, Mercer spent her early years in Victoria. Her mother later took her to Italy to complete her education, and at 17, Mercer ran away to Paris. There, she immersed herself in the Bohemian artist scene of Montparnasse, mingling with prominent figures like Pablo Picasso and members of the School of Paris, including Marc Chagall, Marie Laurencin, Jules Pascin, and Kees van Dongen. In 1922, she worked as a studio assistant at L’Académie Lhôte, translating André Lhôte’s teachings for English-speaking students and absorbing his Synthetic Cubist principles of dynamic symmetry.
Between 1920 and 1922, Mercer and her partner, American artist Alexander Robinson, built a large house in Cassis, France, where they relocated in 1922. Mercer created oil and watercolor landscapes inspired by Lhôte’s teachings and amassed works by artist friends such as Chagall, van Dongen, Louis Marcoussis, Jean-Francis Laglenne, Valentine Prax, Roger Du Fresne, and Gino Severini. Many of these works remained in her collection until her death and were later rediscovered by Rollin Schlicht in 1999. Schlicht, whose father Theo had been a childhood friend of Mercer, found them behind a wardrobe in his parents' Wimbledon home. Included was a folio of Mercer’s own oil and watercolor paintings, which the National Gallery of Australia acquired in 2001, including the small oil painting "House and Olive Trees" from the 1920s, believed to depict her Cassis home.
In the 1920s in France, Mercer developed a close relationship with Janet Cumbrae Stewart, a friendship that was rekindled in Melbourne in 1939. She briefly rented a villa on Capri next to Compton Mackenzie, who later referenced her in his satirical 1927 novel "Extraordinary Women." In the Canary Islands, she fell in love with a German photographer, but their romance was interrupted by the Spanish Civil War. Her lover returned to Germany for military service, while Mercer managed to escape and eventually arrived in Tahiti, where she lived for several years. She later moved to an island off Guam, where she met painter Ian Fairweather. Mercer returned to Melbourne in 1938, renting a studio apartment at 539 Bourke Street (the old St James Buildings) and teaching art. Among her students in 1951 were Lina Bryans, who met her at a Fairweather exhibition, and New Zealand painter Colin McCahon.
Mercer attended George Bell’s School for two months in 1938 and remained close to Bell throughout the 1940s. During World War II, she exhibited with the Contemporary Art Society (CAS), though her "decadent" nudes were often relegated to less prominent spaces in galleries. Her work, influenced by Laurencin and Man Ray’s photographs, garnered some positive reviews, notably from painter and critic Adrian Lawlor. Notable among her works is "Birth of Venus (Study in Diagonals)" (1941), which was shown in several exhibitions before being sold at Sotheby’s Melbourne in 2003.
In Melbourne, Mercer also formed friendships with her neighbors David Strachan and Wolfgang Cardamatis, as well as modernist artists William Frater, Arnold Shore, and Lina Bryans. When she returned to Cassis around 1952, she left some of her Australian works with Bryans, who later donated Mercer's major painting "The Virgin of the Rocks" (1943) to the Art Gallery of South Australia in 1984.
A vibrant hostess, Mercer was fluent in French and Italian and began learning Russian in her seventies after arthritis forced her to stop painting. She spent her final years in France, swapping her Cassis villa for a small house in the grounds of a convalescent home in Aubagne, where she passed away on 14 August 1963, at the age of 81. Her work was largely forgotten until 1975, when Victorian art dealer Russell Davis discovered her paintings and her captivating life story, leading to a significant rediscovery of her contributions to art.