Harnick has a Bachelor of Arts degree from York University, and a law degree from the University of Windsor. He was called to the bar in 1977, and worked as a civil litigation lawyer before entering political life. During his time as an MPP, he was a partner in the Malach & Fiddler law firm. Harnick was appointed a Queen's Counsel on July 1, 1992, and has also been a member of the Canadian Jewish Congress and B'nai Brith.
Harnick first ran for the Ontario legislature in the 1987 provincial legislature, losing to Liberal Gino Matrundola by 4,034 votes in the North York riding of Willowdale. He ran in the same riding in the 1990 election, and defeated Matrundola by 834 votes.
The Progressive Conservatives won a majority government in the 1995 provincial election, and Harnick was re-elected in Willowdale by a landslide. He was appointed Attorney General and Minister responsible for Native Affairs on June 26, 1995.
As Attorney General, Harnick presided over a series of cut to legal services enacted by the Harris government. His government's cuts were particularly criticized for streamlining legal services available to women in vulnerable domestic situations. He also reduced civilian oversight of Toronto's police services, and called for a restoration of capital punishment. He did not run for re-election in 1999.
Harnick became a senior advisor for the Jeffrey Group after leaving politics, as well as becoming a partner at the law firm Sutts Strosberg LLP. He has also served as chair of the Police Costing Review committee in Toronto, and has been involved in negotiations relating to aboriginal self-government.