Exactly one year after being acquired by the Crew, Mullins was traded to Toronto FC in exchange for forward Jordan Hamilton, $50,000 in targeted allocation money, and a 2019 International Roster Spot. Mullins made his club debut on July 20, starting and playing 45 minutes in a 3–1 defeat against Houston Dynamo. He attempted two shots before being replaced by Jozy Altidore. On August 10, Mullins scored his first goal for Toronto, helping the Reds to a 1–1 draw against Orlando City. He went on to appear in both legs of the 2019 Canadian Championship Final and appeared three times in the playoffs as Toronto made a run to MLS Cup 2019, although he was an unused substitute in the final. Mullins made 15 appearances and scored three goals in all competitions and had his contract option picked up by the club following the season.