Cullen Diesel Power can trace its heritage back to Hoffars Limited, which was incorporated in the 1920s. Hoffar's Limited was one of the first General Motors (GM) diesel distributors in North America. They became the marine "Jimmy" diesel distributor in British Columbia in the late 1930s.  Hoffar's Limited successfully populated the growing fishing and towboat markets in the area with the rugged and reliable GM Diesel.

By the late 1960s, Hoffar's Limited had become not only a force in the marine industry, but also established as the industrial GM diesel distributor for the inland areas of British Columbia.

Robert J. Cullen started with Hoffars as a marine engine salesman in 1954. Through a succession of promotions, he became the President of the Hoffars company in the late 1960s. In 1974, Robert J. Cullen purchased the company from the Hoffar family and changed the name to Cullen Detroit Diesel-Allison Limited. Under his guidance, Cullen Detroit Diesel-Allison grew until in 1981.  There were 17 locations and over 450 employees stretching from Halifax, Nova Scotia on the east coast of Canada, to 25 employees in the Cullen owned Detroit Diesel distributorship in the state of Hawaii, U.S.A. The company continued to grow in the 1980s in spite of the economic conditions and some required consolidations.

In 1991 the second generation of the Cullen family reorganized the business to allow Robert J. Cullen to retire. R. Allan Cullen and Detroit Diesel Corporation purchased Cullen Detroit Diesel-Allison Limited and formed Detroit Diesel-Allison British Columbia Ltd.

In 2007 the Allison Transmission division formed

a separate company called ABC Transmissions Ltd.

and the Detroit Diesel division changed to be

Cullen Diesel Power Limited.