The right-back has signed with the Rams on a season-long loan deal.
And the 20-year-old has admitted the presence of his former Chelsea youth coach helped push the move to Pride Park.
"First of all, I am happy to be here – hopefully we can have a good year," Tomori told RamsTV.
"I'm here to learn, I'm here to play games and I'm here to improve. I want to make the fans happy this year.
"I had some good years under Jody at Chelsea. We won the FA Youth Cup a few times, the youth Champions League, but overall, he's just a good guy.
"I am excited to be linking back up with him for this year and, hopefully, we can enjoy some more good times together.
"He's an intense guy, but that's because he's a winner. He wants to do well all the time and is very demanding when it comes to himself and working under him was a big factor in my decision to come here."