"Nothing like this will ever happen again, ever," said an exhausted Isner.
"I don't know what to say."
Isner had match point at 59-58, but Mahut fired down an ace to take it back to deuce, before going on to win the game.
At 59-59 and 9:10pm (0610 AEST) with the light fading fast, match officials tried to suspend play, though Isner wanted to carry on.
"Ladies and gentlemen, due to darkness, play is suspended," the umpire announced as spectators yelled "we want more".
A drained Mahut said: "We played for too long, I don't know how many hours we played."
Roger Federer was among a host of tennis stars who expressed their amazement at events on Court 18.
A string of tennis records were blown away as French qualifier Mahut and American 23rd seed Isner came off court, having slugged it out for exactly 10 hours when play was suspended as they ran out of daylight.
The unfinished fifth set itself was longer than the previous longest match ever played.
That match took six hours and 33 minutes.
The shattered record was set at the 2004 French Open, when Fabrice Santoro beat fellow Frenchman Arnaud Clement 6-4, 6-3, 6-7, 3-6, 16-14.
A string of other records also tumbled as Mahut and Isner tried in vain to end the most astonishing contest ever seen in the illustrious history of the All England Club.
Defending men's singles champion Federer thought he had seen just about everything the sport had to offer in a career which has brought him a record 16 grand slam titles.
But the six-time Wimbledon champion admitted he was stunned by the epic nature of the clash.
"This is beyond anything I've ever seen. It's absolutely amazing," Federer said. "I have almost no words anymore watching this.
"I don't know how their bodies must be feeling. This is incredible tennis. Someone has to lose but in this match both will be winners.
"I love this. I know they're maybe not loving this, but I guess this is unheard of in our game.
"It's so impressive to see. When I was watching this I didn't know whether to laugh or cry. It was too much."
