Robert's mother, Lucimarian Roberts, was 88 years old when she died Thursday.
"It is with deep regret and heavy prayer and reflection tonight that I write to tell you of the passing of Robin's beautiful and inspiring mother, Lucimarian," Tom Cibrowski, executive producer of "GMA," wrote in an email addressed to ABC staff Friday morning.
Because Roberts left "GMA" a day earlier than planned, she was able to be with her mother in Mississippi at the time of her death, ABC News said.
The anchor was planning to leave the show Friday to take an extended medical leave to treat her own bone-marrow disease, one of the myelodysplastic syndromes, or MDS.
Even though traveling to Mississippi meant going through the path of Tropical Storm Isaac on Thursday, Roberts and her sister made the journey so they could be with their mother, ABC said.
Roberts' older sister, Sally-Ann, will be her bone-marrow donor for an anticipated transplant.
"Robin arrived home with her sister Sally-Ann, forging through flooded and blocked roads to be with her beloved mother in time to see her," Cibrowski wrote.
According to ABC News, Lucimarian Roberts was the first African-American to head Mississippi's Board of Education. She loved music and wrote a book titled "My Story, My Song," which Robin Roberts collaborated on.
"I sing because the music of the church speaks my soul language," she wrote in the book. "I sing because these songs are tightly woven in to the texture of who I am."
Lucimarian Roberts' cause of death was not announced, and her family is holding an intimate memorial service in her name.