Mackintosh was born in Fort Frances, northwestern Ontario, and was educated at the University of Manitoba. He trained as a lawyer, and was called to the bar in 1988.
As Attorney General, Mackintosh oversaw the extension of children's rights in the legal system, and supported the creation of Cybertip.ca by Child Find Manitoba to report on online predators and child pornography. In addition, Mackintosh has overseen new initiatives in aboriginal and community justice, and has made efforts to target organized crime in Manitoba. On November 1, 2004, Mackintosh's department announced that it would introduce tough anti-gang legislation, which would permit the province to confiscate the assets of suspected gang members even if no criminal convictions have occurred.
The New Democrats were re-elected in 2003, and Mackintosh was returned in St. Johns with over 72% of the vote.
In September 2006, Mackintosh became Minister of Family Services and Housing, replacing Christine Melnick. Mackintosh assumed the portfolio during a period when the government was facing criticism over a perceived lack of accountability and proper financial controls over the funding of social service agencies. The controversy was sparked when an audit of the agency Hydra House by the provincial auditor found examples of widespread mismanagement and misapplication of public funds.