By capturing the British Open and the PGA Championship, the Dublin native is the first European golfer since England's Nick Faldo in 1990 to earn the honor.
The PGA of America said based on its mathematical formula, Harrington's 116-point total cannot be beaten by his closest challengers in the season's four remaining tournaments.
The announcement was made at the site of the 26th annual PGA Grand Slam of Golf, played at the Mid Ocean Club in Tucker's Town, Bermuda.
Nine-time winner Tiger Woods finished in second place with 78 pints, even though his season ended in June following his dramatic U.S. Open playoff victory against Rocco Mediate because of reconstructive left knee surgery.
Vijay Singh (62 points) was third, with American Phil Mickelson and Spain's Sergio Garcia sharing fourth place with 54 points.