Cousens was elected to the Ontario legislature in the 1981 provincial election, defeating LiberalAlf Stong in the riding of York Centre. He served as a backbench supporter of Bill Davis's government for four years. 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. Cousens was re-elected in the 1985 provincial election, but the Progressive Conservatives were reduced to an unstable minority government. He was appointed Minister of Correctional Services on May 17, but accomplished little of consequence before the Miller government was defeated in the house in June. Along with other members of the Tory caucus, he moved to the opposition benches as Liberal leaderDavid Peterson became Premier. The Progressive Conservatives were reduced to only 16 seats in the 1987 provincial election. Cousens defeated Liberal candidate Gail Newall by fewer than 1,000 votes in the new riding of Markham. In the 1990 provincial elections, Cousens scored a victory over Liberal Frank Scarpitti, winning by more than 10,000 votes. The Progressive Conservatives as a whole managed only a modest recovery, however, increased their caucus size from sixteen to twenty. He served as his party's critic for Environment, Finance, and Citizenship, Race Relations and Human Rights at various times in next parliament. Interviewed by the CBC in 1988, he spoke against voting rights for people who lived in mental health facilities. He criticized Liberal Member of Provincial ParliamentJohn Sola in 1991, after Sola made comments about Canadian Serbs that were generally regarded as racist. Sola was later expelled from the Liberal Party. Cousens once brought forward a private member's bill to outlaw smoking in public places. A Red Tory by inclination, Cousens played little role in the party's drift to the right under Mike Harris. He resigned his seat in the legislature on September 30, 1994 to campaign for mayor of Markham.
Cabinet positions
Municipal politics
Markham is considered a diverse community with a thriving economy, and grew significantly under Cousens's watch. During that time, the town was debt free and gained many jobs in the software field. Unlike Mike Harris's provincial government, Cousens was a supporter of photo radar to discourage speeding in the Greater Toronto Area. There is a Don Cousens Charitable Foundation within Markham. In 2003, he was award a Queen Elizabeth II Golden Jubilee Medal. 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. Donald Cousens Parkway in Markham is named after him. There is also a public school in Markham, Donald Cousens Public School, named after him.