Theo Dimson, growing up with comic books and magazines, became interested in design at an early age. With a scholarship, Dimson attended the Ontario College of Art and Design and graduated in 1950. After graduation, he began a three-year apprenticeship with Art Associates Limited in Toronto and freelanced for seven years before rejoining the firm in 1960 as vice-president of creative design. In 1965, Dimson became president and director of a new partnership Reeson Dimson & Smith Ltd

Dimson won awards and international recognition with exhibits in Finland, Poland, Germany, Japan, Switzerland, and the United States.
He was the author of Great Canadian Posters and was commissioned by Canada Post to create stamps. He had solo shows at Marci Lipman Graphics in Toronto and at the Royal Ontario Museum. He was awarded the Senior Arts Fellowship by the Canada Council and the award of merit from the Directors Club of Toronto and Montreal.

Dimson was a member of the Royal Canadian Academy of the Arts, Associate of the Ontario College of Art and Design, Fellow of the Society of Graphic Designers of Canada and the Advertising & Design Club of Canada in Toronto.