Mackenzie served as his party's Labour Critic from 1985 to 1990. The NDP won a majority government in the 1990 provincial election, and Mackenzie was appointed as the province's Minister of Labour on October 1 of that year. In 1992, he helped navigate the passage of Bill 40, which introduced anti-scab provisions during strikes and other positions supported by the labour movement. The bill became law on January 1, 1993, and is generally regarded as one of the most labour-friendly bills in recent Ontario history. Mackenzie also introduced legislation dealing with pay equity for nurses and child-care attendants, and extended unionization rights to farm workers. In 1993, he accepted the Rae government's "Social Contract" legislation as necessary for the province. In office, Mackenzie was known as a passionate supporter of labour interests within cabinet. Journalist Thomas Walkom recounted the following story in his book, "Rae Days":  There was some controversy in early 1994 when Mackenzie's son David was hired as a special advisor to the Premier's office. Mackenzie resigned as Labour Minister on October 20, 1994, and did not seek re-election in 1995. In 2004, Mackenzie supported Andrea Horwath's successful campaign to replace Agostino in a by-election. Mackenzie died on January 17, 2011 at the age of 82.