In a statement to TODAY, the legendary actress and singer, 71, called Preston — who was married to Newton-John's "Grease" co-star John Travolta for nearly 30 years — a "gentle" spirit and a "devoted" wife and mother.
"Kelly was a radiant, warm and gentle spirit — a loving, devoted wife, mother and sweet friend. My heart breaks for John, Ella and Ben. No words can express my sorrow for her family. Kelly's light shines on in her beautiful children," the statement read.
Newton-John was diagnosed with breast cancer herself in 1992. Following extensive treatment plans and recovery, she was diagnosed with the disease a second time in 2013, but kept that diagnosis from the public until five years later. She learned cancer had returned for a third time in 2017, metastasizing in her spine.
The "Physical" singer previously opened up to FOX News about her longtime friendship with Travolta. During the interview, she revealed she was also "good friends with his wife."
"We will always be close. We’re always busy, but we also find ways to see each other when we can," said the star.
Travolta, 66, announced Preston's passing early Monday in a heartbreaking post on Instagram.
"It is with a very heavy heart that I inform you that my beautiful wife Kelly has lost her two-year battle with breast cancer," Travolta wrote next to a photo of the "Jerry Maguire" star.
"She fought a courageous fight with the love and support of so many. My family and I will forever be grateful to her doctors and nurses at MD Anderson Cancer Center, all the medical centers that have helped, as well as her many friends and loved ones who have been by her side," he continued.
"Kelly’s love and life will always be remembered," he added. "I will be taking some time to be there for my children who have lost their mother, so forgive me in advance if you don’t hear from us for a while. But please know that I will feel your outpouring of love in the weeks and months ahead as we heal..."