Roy Thomas was born on December 29, 1949, near Pagwachuan Lake, Ontario, and grew up on the Longlac Reserve approximately 160 miles northeast of Thunder Bay. He was raised primarily by his grandparents, who gave him the name Gahgahgeh (Crow) after he rescued and cared for an orphaned crow as a small boy. This crow became a signature element in his artwork.

Thomas began drawing at age six, encouraged by his grandmother who would tell him stories while he traced the images they brought to mind. She taught him to draw using sticks of varying lengths, starting in sand, earth, or snow before progressing to pencil and brushes. At age 13, Thomas experienced a significant tragedy when his parents and grandparents were killed in a car accident. He left school at 15 and worked various manual jobs while traveling.

During his youth, Thomas struggled with alcohol addiction, which he later overcame. At age 16, he met Norval Morrisseau while both were incarcerated in Geraldton. Watching Morrisseau paint on birchbark proved pivotal, as the older artist encouraged Thomas to continue developing his artistic abilities. This encounter helped establish Thomas's path as an artist within the Woodland Art style, of which Morrisseau was the founder.

Thomas had his first solo exhibition at the Nightingale Gallery in Toronto in 1966. The Pollock Gallery in Toronto mounted a solo exhibition of his work in 1977 and continued to feature annual exhibitions of his work until 1980. In 1985, Thomas met Louise while she was working for an organization providing business assistance to artists in Alberta. They married and initially lived in Alberta before returning to the Longlac area following the birth of their twin sons, Roy Jr. and Randy, in 1988.

In 1997, Thomas opened his own studio, with Louise joining to handle the business aspects of his art career. Thomas was diagnosed with cancer in 2001 but continued painting at the studio throughout his treatments. His work is characterized by bold use of line and color, featuring symbolic imagery inspired by traditional Ojibway pictographs and visions he believed were taught to him by elders and the spirit of art.

Thomas's paintings have been exhibited at major institutions including the National Gallery of Canada, the Canadian Museum of History, and the Art Gallery of Ontario. The Thunder Bay Art Gallery organized a major traveling retrospective titled "Vision Circle: The Art of Roy Thomas." His work is represented in collections across Canada, the United States, Europe, and Japan, including the Canadian Museum of History and the Smithsonian National Museum of the American Indian.

Following Thomas's death, Louise converted their studio into a gallery to promote the work of Indigenous artists, including their son Randy and other local artists. Thomas is recognized as one of the most influential artists of the second generation of Woodland Art practitioners, contributing significantly to this artistic movement that emerged among Anishinaabe artists in northwestern Ontario during the 1960s and 1970s.