Lawrence left United at the end of last season and his appointment by Newcastle was confirmed this week. He will be assisted by Robbie Stockdale, who has been in interim charge of the Under-21s side since the departure of Diarmuid O'Carroll.
Lawrence said: "It's a real privilege to join Newcastle United and to lead the Under-21s squad at such an exciting time for the Academy.
"I've always believed in creating environments where young players can grow - not just as footballers, but as people - and that is something the Academy does very well.
"The club has a clear vision for development, and I'm looking forward to working with the staff and players to help each individual take the next step in their journey."
Academy Director Steve Harper told the club's website: "We are delighted to welcome Adam to Newcastle United as new Under-21s Lead Coach.
"We have carried out a thorough recruitment process to ensure we find the right person to nurture and develop our young players, and in Adam, we have an outstanding young coach joining the club.
"His record speaks for itself but results only tell part of the story. Throughout his career, he has prioritised the individual development of the young players he has worked with, and he will play a key role in supporting our talent pathway into the first team.
"I'd also like to thank Robbie Stockdale and everyone connected with the Under-21s for their hard work and collaborative approach while we recruited into this position."