Antuna scored the go-ahead goal in the 50th minute, breaking a tie that lasted just two minutes, after an unmarked Lassi Lappalainen headed in a cross from Impact teammate Saphir Taider.
Ibrahimovic was in the right place at the right time in the 31st minute for his fourth goal in two games. Cristian Pavon -- who played a key role in setting up both Galaxy goals -- laid a pass back to Jonathan Dos Santos, who took a shot from the top of the box. It caromed off Antuna and deflected to Ibrahimovic, who had a clear shot into the net.
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The winning goal came after Pavon recovered a loose ball from Bush in the right corner. He played a cross to Ibrahimovic, who headed it down to Antuna, who slotted it home from close range.
Rudy Camacho's header in the 68th minute rolled just wide of the net as Montreal, with a sense of urgency, ramped up the pressure in the second half.
"I'm the MVP of the MVP's," Ibrahimovic said after the game. "I don't focus on that. I focus to perform and to make my team win. Now, we're in a good position, we just have to continue. This is what we want, we want to be in the playoffs and in the end, we will take the rest from there."
The Galaxy (15-13-3, 48 points) moved into third place in Major League Soccer's Western Conference for the moment. Minnesota United can retake that spot with one point from a draw or three from a win on Sunday in Portland, as teams jockey for playoff position in the final weeks of the regular season.
The Impact's loss (11-17-4, 37 points) was likely the knockout blow to their playoff hopes in the Eastern Conference. They suffered their third straight loss and trail the New England Revolution for the seventh and last postseason berth by four points with only two games left to play.
The Galaxy have moved from eighth place to third in the span of two games, both wins. Ibrahimovic remained two goals behind the league's leading scorer, Carlos Vela of Los Angeles FC, who buried his 29th on a stoppage-time penalty kick Saturday night.
Ibrahimovic alone had eight of the Galaxy's 12 shots on goal. Montreal goalkeeper Evan Bush had 10 saves. David Bingham made one for the Galaxy.
Zlatan scores and supplies in Galaxy victory
Zlatan Ibrahimovic provided a goal and an assist in the LA Galaxy's 2-1 win over the Montreal Impact. To watch MLS, sign up for ESPN+.
Zlatan in the right spot for Galaxy's opener
Major League Soccer: Zlatan Ibrahimovic (31') LA Galaxy 1-0 Montreal Impact. To watch MLS, sign up for ESPN+.