Johnson signed with the Mavericks two months ago with the understanding he would serve as both a player and coach. Dallas Coach Don Nelson said at the time he hoped Johnson would succeed him.
"I am looking forward to working with a Hall of Fame coach in Don Nelson, who has been so good to me and supportive in helping me maximize my coaching abilities," Johnson said.
"I am especially grateful to (Mavericks owner) Mark Cuban for being a man of his word and a man of great integrity. I want to be a part of him and this organization winning their first championship."
Johnson began his career as a free agent with Seattle in 1988. He is one of 75 players to have taken part in 1,000 NBA games, having also played with Denver, San Antonio, Houston and Golden State.
He was the starting point guard on San Antonio's championship team in 1999.