He was a Progressive Conservative member of the Legislative Assembly of Ontario from 1981 to 1994, and briefly served as a cabinet minister in the government of Frank Miller.
He was also an officer in the Canadian Forces (Commandant in the Fort Henry Guard), he then served as an executive from 1967 to 1992 (firstly with Honeywell Ltd.) before entering political life, and later became a chair of the York Technology Association.
Cousens initially supported Dennis Timbrell to succeed Davis in the Progressive Conservative Party's 1985 leadership convention, but crossed over to Frank Miller on the last ballot.
He was named parliamentary assistant to the Minister of Citizenship and Culture on February 25, 1985, shortly after Miller was sworn in as Premier of Ontario.
He served as his party's critic for Environment, Finance, and Citizenship, Race Relations and Human Rights at various times in next parliament.
Unlike Mike Harris's provincial government, Cousens was a supporter of photo radar to discourage speeding in the Greater Toronto Area.
On June 1, 2006 Cousens announced that he would not seek re-election as Mayor of Markham due to on-going health problems related to the need for a kidney transplant.