The 30-year-old native of New Westminster, B.C., was honoured Thursday during the CFL's awards banquet at the Queen Elizabeth Theatre.
Cornish claimed the award in voting conducted by the Football Reporters of Canada and nine CFL head coaches.
The six-foot, 209-pound Cornish led the CFL in rushing with 1,082 yards and averaged 7.8 yards per carry despite only playing nine games due to injuries.
Defensive tackle Ted Laurent of the Hamilton Tiger-Cats, who recorded a career-best nine sacks, was the finalist.
Calgary and Hamilton will meet Sunday in the Grey Cup game.