Despite a stiff wind Harrington followed a four at par-5 second with birdies at the next two to be alongside Tom Kite.
Harrington then took a two-and-half-hour break with the rest of the strong field as rain whipped across the Sawgrass course, but came out again all guns firing.
His approach to the 7th with a wedge from 150 yards found the middle of the green and he rolled in the putted to go four under par and a two stroke lead over a group of players including Kite - who bogeyed his 10th hole -Glen Day, Brad Faxon and Kenny Perry.
Colin Montgomerie birdied the 10th to go one under par.
Montgomerie, given polite applause by the 20 spectators on the first tee for his 7.10am start, parred the first six holes.
Jose-Maria Olazabal birdied the second, but then had bogeys at the fifth, sixth, ninth and 10th.
Lee Westwood made mistakes at the fifth and eighth, while playing partner Langer double-bogeyed the sixth, before getting back to even par with successive birdies.
Title-holder Tiger Woods faces a delayed start at 10pm UK time