After graduation he worked for the Great West Life Assurance Company and subsequently, Richardson Securities of Canada as a corporate underwriter.
Within a few years of taking over Harlequin Enterprises from his father, he moved the company's operations to Toronto, Ontario, where he would build it into a major international force in the book publishing industry.
In addition, he oversaw expansion to Australia in 1974, The Netherlands in 1975, set up a joint venture in 1976 in West Germany and in 1977 established a subsidiary in France.
Bonnycastle also published 4 books: To Ride the Wind – Albert Hochbaum, A Gentleman Adventurer, I Fought Reil, My Life with Birds – Angus Shortt.
Bonnycastle supported various causes related to nature and the environment and has been a council member of Fort Whyte Centre for Environmental Education (1988–2015), trustee of the Institute for Wetland and Waterfowl Research (1990–1995), co-founder of the Canadian Parks and Wilderness Association (1988), trustee of the North American Wildlife Foundation, operating Delta Waterfowl and Wetlands Research Station (1978–1988).