Hearn went on to enter federal politics and was a member of the Conservative Party of Canada in the House of Commons of Canada, representing the riding of St. John's West from 2000 to 2003 and St. John's South—Mount Pearl from 2003 to 2008. He was a member of the Progressive Conservative Party of Canada from 2000 to 2004, and was active in representing the party during its merger discussions with the Canadian Alliance. Those discussions culminated in the merger of the two parties in December 2003, to the Conservative Party of Canada. Hearn served as the first House Leader of the newly created party until it had its first leadership convention.