Miami started with all their big-name stars, including Lionel Messi, on the bench but the Argentine was introduced in the 61st minute to the delight of many in the Toronto crowd.
The Florida side had already secured the Supporters' Shield, for the top team in the regular season, with their midweek win at the Columbus Crew.
With top seed and home field advantage secured for the playoffs, the only target left for Miami is to try to better the record of 73 points in a season set by the New England Revolution in 2021.
The win leaves Miami on 71 points ahead of the final game of the season -- at home to New England on October 19.
An uneventful game, on a less than perfect field, was settled in added time when Luis Suarez, also on as a substitute, pumped a cross into the box and Ecuador international Campana took the ball on his thigh before volleying home on the turn.
It was only the second shot on target of the game for Miami and it sorely damaged Toronto's slim hopes of making the post-season.
John Herdman's team currently sit in ninth place in the Eastern Conference, which would give them a place in the wildcard round of the playoffs.
But the defeat was Toronto's last game of the season and with rivals Philadelphia and D.C. United level with them on 37 points and with two games left, the Canadian team's season looks over.
Philadelphia travel to defending MLS champions Columbus later on Saturday while D.C. are away to New England.
