Cornish was named the league's top offensive player and top Canadian after he rushed for 439 yards and two touchdowns and added 62 receiving yards and a touchdown over three games.
The Stampeders were 3-0 with Cornish in the lineup in September. He was injured and didn't dress for Calgary's 31-15 Week 15 loss in Montreal.
Montreal linebacker Bear Woods won defensive player of the month while Saskatchewan receiver/returner Weston Dressler won the special teams award.
Cornish, who averaged 9.15 yards per carry in September, had standout performance in Week 12. The 29-year-old from New Westminster, B.C., registered a season-high 174 yards on 11 carries, including a career-long 79-yard touchdown run, to help the Stampeders to a 40-33 comeback win in Toronto.
Cornish won the Canadian player of the week all three times he played last month and added an offensive player award for Week 12.
Cornish is 21 yards behind CFL leading rusher Brandon Whitaker of Montreal despite missing seven games this season.
Woods recorded 33 tackles, three sacks and forced a fumble in the Alouettes' four games September.
He had a career game in Week 11 with personal bests in both tackles (10) and sacks (two) in a game as the Alouettes prevailed beat visiting Hamilton 38-31.
Dressler returned to the CFL in Week 10 and made an immediate impact for the Riders. In four games in September, Dressler returned two punt returns for touchdowns and averaged 14.7 yards per return.
In Week 11, Dressler's 75-yard punt return for a touchdown proved to be the eventual game-winner as the Riders beat Winnipeg 30-24. In Week 13 against Ottawa , he returned a punt 78 yards for a touchdown to establish a new career-high.