Wilson netted seven times in 13 Championship matches for Hull City in the second half of the previous campaign, while Mount scored 14 goals and assisted another nine to be crowned Vitesse Arnhem's Player of the Year.
Lampard is glad to have agreed terms for the Premier League duo, telling County's official website: "I'm delighted to get both of these deals done and add players of their quality and calibre to the squad. I know Mason really well from my Chelsea links and I have also watched him play in their very successful youth teams.
"I knew him from those days and I watched him closely at Vitesse, where he took it by storm. His numbers are incredible for an attacking midfield player of his age and it's a massive coup for me to get him here. He can bring us creativity in the attacking midfield area, goals, assists as well as energy.
"He was wanted across Europe by top, top, clubs and we should be very happy that we have got him. The thing about Mason is it's not just on the pitch. He's so dedicated and hungry off it too. He wants to do well, and he wants to improve. This is a real stage to develop for him and to go out and play at a really competitive level with us.
"Harry brings incredible quality and hunger to the table here and he showed that last year while he was on loan at Hull. He's a full international too and he has a real desire to come here and improve. He's going to be a crucial player for us. I keep talking about creativity and the importance of trying to excite the fans and he's the sort of player that can do that with the quality that he has."
Derby are also said to be closing in on a loan move for Manchester United defender Demetri Mitchell.