Netball Australia has decided not to renew her contract.
Alexander was contracted until the end of 2020 but she will now leave after coaching the Diamonds one final time at the bushfire relief charity match in Sydney on March 1.
The Diamonds have long been one of Australia's most successful national teams but last year they lost the World Cup title for the first time since 2003 and they also lost the Commonwealth Games gold medal match to England a year earlier.
Netball Australia CEO Marne Fechner paid tribute to Alexander but said there was an "appetite for change".
"Lisa has had a significant impact on our sport and achieved great things in her time, both on and off the court. She is leaving a great legacy," Fechner said in a Netball Australia statement.
"As we looked to the next high-performance cycle and beyond, there was a consensus and appetite for change heading into 2021.
"The high-performance landscape is ever changing and increasingly competitive and we believe it's in the best interest of the program to open up the role."
Alexander admitted she'd hoped to continue.
"It has been an honour and a privilege to serve as the Australian Diamonds head coach for the past eight-and-a-half years," Alexander said in the statement.
"I would dearly have loved to continue in the role beyond December 2020, and whilst disappointed with the decision not to renew my contract, I respect the decision of the board to head in a new direction.
"I will not be applying for the position and I wish the athletes, support staff, high performance staff and new coach all the very best for the future and for continued success and excellence for the Australian Diamonds."
Netball Australia will begin recruitment for the next Diamonds coach in early next month.
