The Impact moved back to the top of the East while Chicago's winning ways were short-lived.
Andres Romero scored the first goal in the 57th minute. The on-loan Argentine collected a ball sprayed wide by Andrea Pisanu, cut inside Logan Pause, and rifled a shot into the far corner.
Chicago's woes were compounded by a red card to Jeff Larentowicz in the 64th minute. The veteran Fire man was judged to have clipped Pisanu just outside of the penalty area.
Marco Di Vaio gave Montreal a cushion in the 75th minute. New signing Daniele Paponi did well to carve out a chance for his countryman, and Di Vaio came up with a tricky first touch, turn and shot into the corner of Sean Johnson's net for his team-leading fourth of the season.
The result gives Montreal a sterling 5-1-1 record for the season, while Chicago limps into May carrying a 2-5-1 showing.