And Hartley admits that the 21-year-old Welsh flyer has already made quite the impression on his new teammates.
"He's so professional in the way he trains and I think that rubs off on the other players," Hartley told Wales Online.
"What age is he? 21? For such a young lad his impact on the training field and in team meetings is considerable.
"He's not young in that way and he certainly adds to the meetings - he's settling in very well and fitting in."
The 31-cap winger has started all four of Northampton's league games so far this season, getting on the scoresheet for the first time in the 33-14 victory over Sale Sharks at the weekend.
And Hartley isn't the only one thrilled with the Wales star's influence on the pitch, with Northampton director of rugby Jim Mallinder also singing his praises.
"George and our other new signings have added to the mix," said Mallinder.
"We've spread our load a little better and made some good signings.
"You have just got to see George's contribution, he's not had 100 ball carries or made 100 tackles, but what he has done has been pretty impressive.
"What you want is players like that to produce big things in big games.
"I don't know what impact signing George had on our season-ticket sales but what I do know is the fans have taken to him.
"There's a real good buzz around Franklin's Gardens at the moment."