He operated a private legal practice in Toronto from 1965 to 1966, when he joined the firm of Beatty, Bowyer & Greenslade in Brampton.
Callahan ran for the Ontario legislature in the 1977 provincial election, but finished third against Progressive Conservative Bill Davis, the sitting Premier, the riding of Brampton.
In the provincial election of 1985, the Progressive Conservatives under Frank Miller were reduced to a minority government as Liberal support increased in much of the province.
He chaired the Public Accounts Committee of the Legislative Assembly, which studied alcohol and drug addiction, and treatment.
"[2] During this term, Callahan served in a variety of roles:[8] The New Democratic Party won a majority government in the 1990 provincial election, and the Liberals were reduced to only 36 MPPs.
During this term, Callahan served in a variety of roles:[8] In the 1995 provincial election, the Progressive Conservative Party under Mike Harris won a majority government based primarily on support from Greater Toronto Area communities such as Brampton.
[11] Callahan was gracious despite his loss, as he would often visit Clement's constituency office to suggest ideas and give updates on projects.