He was a Progressive Conservative member of the Legislative Assembly of Ontario from 1990 to 1999, and served as a senior cabinet minister in the government of Mike Harris.

Harnick was certified as a Specialist in Civil Litigation in 1991; he was appointed a Queen's Counsel in 1992 and in 2005 he was awarded the Law Society Medal.

In June 1996, Harnick was asked in the legislature to comment about a racist remark that Premier Mike Harris had allegedly made regarding the Ipperwash crisis.

Harnick is a founding Principal of Counsel Public Affairs Inc.[6] He is a former board member and Vice Chairman of St. John's Rehabilitation Hospital and he is an ex officio Bencher of the Law Society of Upper Canada.

He served as chair of the Police Costing Review committee in Toronto, and has been involved in negotiations relating to aboriginal self-government.