He began his career at Bell Canada in 1974 and held various positions in the BCE group, including vice-chairman and CEO of Northern Telecom Limited, before rising to become president and CEO of BCE, Inc. and chairman and CEO of Bell Canada. By 1998, he was a director of BCE, Inc., BCE Mobile Communications Inc., Telesat, Bell Satellite Services, Bell Canada International, Inc., Tele-Direct Inc., Bombadier Inc., Northern Telecom Limited, Teleglobe Canada Inc., SNC-Lavalin, the Bank of Montreal, and the Lagardère Group.

Monty chaired the Canada Millenium Scholarship Foundation and served as a member of the Executive Committee of the Business Council on National Issues. The Academy of International Business named Monty International Executive of the Year in 1996, and both Caldwell Partners and the Financial Post named him Canada's Outstanding CEO of the Year for 1997. Monty was awarded the highest civilian honor that the Government of Canada can bestow — the Order of Canada — for his contributions to business, policy, and community affairs, in 1995.