After graduation, Dimson, began a three-year apprenticeship with Art Associates Limited in Toronto. He 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. The company later became Dimson & Smith Ltd. and maintained this name until Dimson created Theo Dimson Designs Inc. in 1985. Dimson won awards and international recognition with exhibits in Finland, Poland, Germany, Japan, Switzerland, and the United States. His works have been published in Canada Art, Creativity Magazine, Studio Magazine, the Toronto Star, the Globe and Mail, Art Direction Magazine, Idea Magazine (Japan) and Graphis Magazine (Switzerland).

He is the author of Great Canadian Posters and was commissioned by Canada Post to create numerous stamps. Dimson had a retrospective show of his design work at the Royal Ontario Museum in 1993-94. 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, Alliance graphique international, and the Advertising & Design Club of Canada.