Encouraged by fellow designers in a Toronto commercial-art firm, Thomson began to paint about 1911. In 1913 he and his colleagues (including A.Y. Jackson and J.E.H. MacDonald) went to Algonquin Provincial Park to paint. After this trip Thomson spent part of each year in the park as a woodsman, guide, and painter. His pictures depict lakes, mountains, and trees vigorously painted in textured patterns of brilliant colour. They provided an impetus to the formation of the Toronto-based landscape painters originally known as the Group of Seven (Seven, Group of).