Toronto clinched a playoff spot with the victory and extended its lead atop the MLS standings. Toronto FC is on a five-game winning streak and is 6-0-1 over their last seven games.
FC Cincinnati is now scoreless over their last 455 minutes of play, though not without a couple of close calls in stoppage time on Sunday.
In the 94th minute, a penalty kick was initially awarded to FC Cincinnati after Toronto defender Omar Gonzalez was called for a foul in the box. A video review, however, reversed the referee's call.
A minute later, Toronto almost conceded an own goal after a wayward back pass from defender Laurent Ciman.
Cincinnati is 0-4-1 over its last five games, and has lost four games in a row. Sunday's result was the club's first loss (0-1-4) in five home games this season, and the first to not finish in a 0-0 draw.
Adding to FC Cincinnati's woes, defender Mathieu Deplagne (hip) and goalkeeper Przemyslaw Tyton (leg) both left the game due to apparent injuries.
Toronto goalkeeper Quentin Westberg made one save for his fifth shutout of the season.
After a quiet opening 28 minutes for both teams, Toronto FC struck in the 29th. Richie Laryea took the ball almost to the goal line before passing back through the box to an open Mullins for the strike.
Mullins scored for the first time since Sept. 7, 2019, also against FC Cincinnati.
Mullins was one of several Toronto bench players getting an opportunity in the starting 11. Due to both a congested schedule and consecutive games on an artificial surface, Toronto FC made seven changes from their starting lineup in last Wednesday's 1-0 victory over the New England Revolution.
Laryea was substituted out of the game in the 70th minute with a possible hamstring injury.