He was born in Mallow, Ireland and as a young boy he immigrated to Canada with his family sometime before 1822.
In his early career he worked as a furniture decorator. He was inspired by the American Indian paintings of George Catlin. In 1845 with the help of Sir George Simpson of the Hudson's Bay Company he set out with the fur-traders to document the Canadian west. He was present at the last great buffalo hunt. After four years he returned to Eastern Canada with over 700 sketches. Mount St. Helens, the volcano in the present-day State of Washington, was one of his subjects.
Paintings based on his sketches were made after his return to Toronto. He also wrote of his experiences in his book "Wanderings of an Artist - Among the Indians of North America" published in 1859.
He died in Toronto, Ontario, Canada.
On February 25, 2002, his portrait of Jon Leffroy garnered a record price for a painting by a Canadian painter, C$4.6 million.