Collen Warner was dismissed for a handball on the line, setting the stage for a convincing Sporting victory in Quebec.
Montreal went down a man in the 17th minute when Collen Warner handled the ball on the line in a goalmouth scramble, and was shown a red card. Dom Dwyer casually stroked home the ensuing penalty kick, and it was all downhill from there for the home side.
Paulo Nagamura knocked in the second goal in the 34th minute. Unmarked on the far side of the box, he drifted in and connected with Chance Myers' cross, his shot taking a deflection as it beat Troy Perkins.
Dwyer picked up another in the second half, collecting Antonio Dovale's through pass and picking his shot just inside the post to complete the 3-0 scoreline in the 64th minute.
Kansas City moves into full possession of the top spot in Major League Soccer's Eastern Conference with a 5-2-2 record. Montreal lags back in ninth, with a dismal 1-5-3 showing on the year so far.