Lomas has added the club's record goalscorer to his backroom team after he announced his retirement from playing last week.
Lomas told the club website: "As soon as I became aware that Neil's playing career was coming to an end I was keen to bring him onto the staff at Millwall once more.
"In the same way as I'm looking for players hungry to play for this club who understand its dynamic, the same goes for the coaching staff.
"I see Neil in the same position I was when I finished as a player and had the opportunity to get my coaching career under way at Norwich City.
"I believe he will be a great asset to us and an inspiration to the players, both at first-team level and with the development squad."
Harris played for Southend in League Two last season but has been forced to hang up his boots due to a knee injury.
The 35-year-old added: "I'm so excited to be coming back to the club where I enjoyed so many happy years as a player.
"The opportunity to pass on my knowledge and experience to the current generation of Millwall players and start my coaching career at the place I love is something I could only have dreamed of as I contemplated the fact that my playing days were over."