He was elected to the Ontario legislature in the general election of 1977, defeating Liberal Michael MacDougall by over 2,000 votes in the riding of Windsor—Riverside. He was re-elected by greater margins in the elections of 1981, 1985, 1987 and 1990. Cooke defeated future Liberal MPRick Limoges by almost 6,000 votes in 1987. Unlike most other members of the NDP caucus, Cooke was supportive of Bob Rae's leadership throughout the 1980s. As early as 1981, he had petitioned Rae to run for the leadership of the provincial party. He became the party's whip in 1985 and acted as the health critic. In 1987, he was appointed as the party's House leader, and became pensions critic.
In government
The NDP won a majority government in the 1990 election. Cooke, as one of the party's most senior MPPs, was appointed Minister of Housing and Municipal Affairs on October 1, 1990. On July 31, 1991, he was appointed Government House Leader and he moved to the Ministry of Municipal Affairs. Cooke also served as Chair of the Management Board of Cabinet from September 23, 1992 to February 3, 1993. As Minister of Housing and Municipal Affairs, Cooke announced that the government would restrict most rent increases to 4.6% for the present year and 5.4% for 1991. The provisions for 1990 were made retroactive. Tenants' groups supported these changes, while landlord representatives were generally opposed. A few days after the policy announcement, a landlord group placed a $25,000 advertisement in The Wall Street Journal suggesting that investors avoid Ontario. Their ploy was widely criticized. Cooke later announced that he would work to factor in the costs of legitimate building renovations. On February 3, 1993, Cooke was promoted to the high-profile position of Minister of Education, which he retained for the remainder of the Rae government's time in office. Cooke endorsed the Rae government's establishment of a casino in Windsor as a means of stimulating the city's economy during the recession of the early 1990s. He also strongly opposed cuts to welfare recipients. As education minister, he maintained support for "de-streaming" and endorsed and implemented province-wide testing.
Cabinet positions
Opposition and resignation
The NDP lost the 1995 provincial election, although Cooke was re-elected in Windsor—Riverside with a reduced majority. He continued to serve as NDP house leader from 1995 to 1996. In 1996, he endorsed Frances Lankin's unsuccessful bid to replace Rae as party leader. Cooke resigned from the legislature in January 1997, after being appointed to an "Education Improvement Commission" established by the Progressive Conservative government of Mike Harris, later chairing the commission.
After politics
He was a regular member of TVOntario's Studio 2political discussion panel. He was head of the Board of governors at the University of Windsor until 2009. In 2008, he was appointed by Windsor mayor Eddie Francis to chair a committee to examine the feasibility of a marina-canal plan in the city's downtown area. The concept was eventually rejected by city council as too expensive. In February, 2015, Cooke was named Chair of Ontario's Education Quality and Accountability Office. He became a member of the EQAO Board of Directors in 2008.