However, in the later starters, Jordan Spieth and Rory McIlroy showed they were over the disappointment of the US Open last week where both missed the cut.
Both McIlroy and Spieth were tied for the lead at Cromwell, Connecticut, after 16 holes. McIlroy compiled seven birdies through 16 holes, including three in a row from 13-15..
Harrington had four birdies on his card with no dropped shots at TPC River Highlands, which at 6,848 yards, is one of the shortest on the PGA Tour.
Seamus Power shot a level-par 70 to lie in a tie for 79th while Graeme McDowell had a disappointing start, after a two-over par 72.
Zach Johnson has shot a first-round 63 to take an early lead at the Travelers Championship in Connecticut.
The former Masters and British Open champion birdied six straight holes on the back nine and finished 7 under par, one shot ahead of American Peter Malnati.
Brooks Koepka shot a 2-under 68 – impressive considering he’s gotten little sleep since winning his second straight US Open on Sunday. He was at 4 under until suffering two bogeys in his last three holes.
“I just ran out of gas,” said Koepka, whose only prep work this week was a range session Wednesday. “I’m exhausted mentally. I’m excited to go home and nap.”
Koepka didn’t return to his home until early Monday morning. A few hours later, Dustin Johnson came over. “Dustin was in my living room at 8. He came over on the boat to say hi,” Koepka said. “So It was not as much rest as I would have liked.”
On the European Tour, Frenchman Sebastien Gros coped best with testing windy conditions to take the lead after the first day of the BMW International Open.
With more than three quarters of the field over par at Golf Club Gut Laerchenhof in Pulheim, near Cologne - and no player bogey-free - the 28-year-old’s one dropped shot at the short 16th was impressive and he reached the clubhouse on four under.
Gros was four under for his outward nine and added a fifth birdie at the par-five 15th, slipped up at the next and missed a 10-foot birdie chance at his penultimate hole.
It left him ahead of a group including European Tour rookie pair Aaron Rai and Henrik Sturehed, Scotland’s Scott Jamieson and Spaniard Jorge Campillo, who bogeyed his last hole - the ninth - to drop out of a share of the lead.
“It was quite brutal with the wind and it was tough to get the ball on the fairway,” Gros told Sky Sports.
“I was four under after nine but there was still 63 holes to play. I was just thinking about trying to make the best round I could.
“I’m just one putting week from something very good.”