The Blues entered the new campaign with a dilemma between the sticks following the poor form of Kepa Arrizabalaga.
The 26-year-old made a string of high-profile errors and Lampard moved to replace him with £22million signing Mendy from Rennes.
Since becoming No.1, the Senegal international has transformed Chelsea’s backline and conceded just two goals in his first nine games for the club.
And Lampard hailed the goalkeeper for ‘seamlessly’ transitioning into the first team following his summer move.
‘I’ve been really pleased,’ Lampard told Chelsea TV. ‘It’s not easy and he’s made it look easy, which is the difficult thing.
‘When you come from another league there are of course differences in the Premier League – the physical nature, the speed of the league – and I think it is unique in those terms.
‘For a goalkeeper that has got such a spotlight on him and is having to face these differences and make relationships with the team around him, he has done it very seamlessly.
‘He has settled off the pitch and is performing on the pitch, you can’t ask any more than that, and he is doing it with a smile on his face and with a really great attitude. So I’m really pleased, it’s a good start for him in a Chelsea shirt. Long may that continue.’
They are also faring well in Europe and can book their place in the Champions League knockout stages if they beat Rennes on Tuesday.