Lieb and Hartley join Webber for Porsche LMP1
Lieb said: "This is something special for me. I was brought up in Stuttgart, my father had been a Porsche apprentice, so as a kid I dreamed of winning the Le Mans 24 Hours with Porsche and not about Formula 1.
"When Porsche announced that it was coming back with an LMP1, I thought this was a once in a lifetime opportunity to try to win Le Mans outright."
Lieb, 33, said that he "immediately felt at home" when he drove the LMP1 for the first time at Paul Ricard, despite a lack of experience in downforce cars.
"It reacts differently to a GT car, but it is so much fun because it has so much more grip and power," he explained, while conceding that he "still had a lot to learn".
Hartley, who has impressed in his first two years of prototype racing with the Murphy Prototypes LMP2 team in Europe, said he was "incredibly proud that Porsche has chosen me".
"I was deeply impressed by my first outings at Le Mans, but to compete there in LMP1 for an iconic company like Porsche is another dimension completely," he added.
Lieb and Hartley join Porsche regulars and former Le Mans 24 Hours winners (with Audi) Timo Bernhard and Romain Dumas, ex-F1 driver Mark Webber and Swiss Neel Jani.
Porsche research and development boss Wolfgang Hatz, the board member responsible for motorsport, said that Porsche was proud of its driver line-up for next year's WEC.
"Three of the six drivers come from our own Porsche driver squad and two were even Porsche juniors," he said. "This is something we are particularly proud of."