Mackintosh grew up in Fortrose, 50 minutes south east of Invercargill and played for the Tokanui Rugby Club's junior sides.

For the 2007 season, he went two better, and played in ten matches, starting in all of them, and enjoyed wins over Otago, Bay of Plenty and Manawatu.

Having earned 123 caps for the team, the peak of his accomplishments was helping bring the Ranfurly Shield back to Southland in 2009, breaking a drought of 50 years in doing so.

In March 2016, Mackintosh joined the American newly formed PRO Rugby competition with the Ohio Aviators.

He started in two matches for the All Blacks, the 32–6 victory in the test against Scotland and a full 80 minutes in the hard-fought 18–16 win against Munster.