Ms Letourneau's lawyer David Gehrke told USA media that she had died at home surrounded by her children and Mr Fualaau.
Police caught the pair in a auto at a marina parking lot, but no charges were laid after Fualaau's mother told the cops to return him to Letourneau, not knowing that Letourneau lied about Fualaau's age and what actually happened in the vehicle.
The relationship wasn't to last: They filed for a formal separation in 2017.
"Mary fought tirelessly against this bad disease", the statement said.
Mary Kay's childhood was shadowed by tragedy; her younger brother, Phillip, drowned in a pool when he was 3 as she was playing with another brother at the other end. "We did our very best to care for Mary and one another as we kept her close and stayed close together", the statement continued.
Letourneau was charged with rape of a child after Letourneau became pregnant following a sexual relationship with student Vili Fualaau.
She gave birth to their second daughter in prison in 1998.
Letourneau was released from prison in 2004.
In 1997, Letourneau pleaded guilty to rape, and while she was waiting to be sentenced she gave birth to her and Fualaau's daughter, Audrey. She was paroled after six months but was again caught having sex with Fualaau, a violation of her parole conditions. She twice became pregnant with Fualaau's children before he was 15, despite court orders aimed at keeping them apart.
As for Letourneau and Fualauu's relationship: "Everyone said it wouldn't last, but it did, at least for 20 years". They divorced in 2019. LeTourneau was handed the maximum seven-and-a-half-year prison sentence for her crimes.
According to Gehrke, Fualuaa's decision to return to Seattle to care for her "meant to world" to Letourneau.
"Mary was a client, a good friend, a wonderful mother, and a force of nature", longtime friend and attorney Anne Bremner wrote in an email to The Seattle Times.
"She paid her debt to society". She was brilliant and dedicated to her work and family.