Born in Montreal, Kirby earned a Bachelor of Science and Master of Arts in Mathematics from Dalhousie University, where he was a member of Phi Delta Theta Fraternity and a Doctor of Philosophy in Applied Mathematics from Northwestern University.
Kirby worked as principal assistant to the Premier of Nova Scotia Gerald Regan from 1970 to 1973 and Assistant Principal Secretary to Prime Minister Pierre Trudeau from 1974 to 1976. He served as President of the Institute for Research on Public Policy from 1977 to 1980.
Kirby returned to public service in the 1980s as Secretary to the Canadian Cabinet for Federal-Provincial Relations and Deputy Clerk of the Queen's Privy Council for Canada. As such he participated in the federal-provincial negotiations that led to the patriation of the Canadian Constitution in 1982. He was elevated to the Canadian Senate by Pierre Trudeau in January 1984 weeks before the prime minister announced his intention to retire.
He remained active in the private sector serving as vice-president of Goldfarb Consultants from 1984 to 1994 at a period when the polling firm was often employed by the Liberal Party of Canada. Kirby also served as a backroom advisor to the Liberals and frequently appeared on television as a political pundit during the 1980s and 1990's.
Kirby was the principal author of a 2002 report by the committee on, Canada's health care system. The report urged greater private sector involvement in health care delivery and was seen as a rival to the royal commission report on health care released by Roy Romanow. Kirby was criticised for having a conflict of interest in his role in writing the report due to his service on the boards of directors of various private health care companies.
In 2007, he was asked by Prime Minister Stephen Harper to become the first Chair of the newly created Mental Health Commission of Canada, a not-for-profit organization that was created in response to his 2003 Senate report on mental health.