[3] Warner was chosen in February 2007 as the Conservative Party of Canada's candidate for the House of Commons in the riding of Toronto Centre by-election, following the retirement of Bill Graham.

[8] He is licensed to practise in Ontario, Canada and New York State, and was previously a Member of the Brussels Bar.

He was an activist in the anti-apartheid movement and campaigned successfully for McGill and the University of Toronto to divest holdings in companies doing business in apartheid-era South Africa.

[11] He is the co-author of Canadian Law and Practice of International Trade (2nd edition, 1997) with Bill Graham, the retired MP for Toronto Centre, amongst others.

[3][17] Warner regularly speaks publicly at events on, trade policy[18] and competition policy[19] and is invited to comment on current economic developments in news media such as the Financial Times,[14] the Toronto Star,[20] Agence France Press,[21] the Wall Street Journal and the Dow Jones Newswire.