Tomori signed a new five-year contract on Thursday.
Lampard said ahead of today's clash with Bournemouth: "He's come a long way and it's all his own doing because from the moment he came to Derby he showed a work ethic and a desire to listen and improve himself.
"His personality has come out even more now because he was quite quiet when he came to Derby but he's less quiet now.
"Everyone saw his performances in the Championship last season and what he did, winning Derby Player of the Year, but then there were big question marks over him in the summer whether he could do it in the Premier League. He certainly hasn't proved that yet over a period of time but from my point of view, I have full trust in him and I think he's done really well.
"I'm delighted for him but the important thing is he doesn't take anything for granted now he's got that nice five-year contract. It's important now that the hard work starts again."