In the opening round of the first of four PGA Tour playoff events, Harrington took apart a course that is considered to be one of the toughest in the country -- the Black Course at Bethpage State Park.
The layout proved extremely difficult when it hosted the U.S. Open in 2002 and 2009, but 51 players broke par to start the playoffs Thursday.
Nick Watney and Brian Harman shared second place at 65 while Sergio Garcia, who won on the PGA Tour for the first time in four years last week at the Wyndham Championship, shot a 66. Garcia was tied for fourth with Pat Perez, Gary Christian and Jimmy Walker.
Defending champion Dustin Johnson and Rickie Fowler were among those at 67 while the large group at 68 included FedEx Cup points leader Tiger Woods, Phil Mickelson, Zach Johnson, Vijay Singh, Luke Donald and British Open champion Ernie Els.
World No. 1 Rory McIlroy, who played in the same group with Zach Johnson and Woods, shot a 69.
Harrington's career has been highlighted by a burst of three victories in a span of six major championships during 2007 and 2008. He will not earn enough points this year to qualify for the Ryder Cup matches and hopes to be one of the two wild card picks announced by European captain Jose Maria Olazabal next week.
The Ryder Cup will be held in suburban Chicago next month.
The top 125 points earners in the year-long race for the FedEx Cup and its $10 million first prize were eligible to play this week.
That number will be cut to 100 for next week's Deutsche Bank Championship and then to 70 for the BMW Championship the following week.
Only the top 30 will survive to compete in the Tour Championship, where the FedEx Cup winner will be decided.