A family representative confirmed her passing in a statement to People magazine, saying her death came after two years fighting the illness.
"Choosing to keep her fight private, she had been undergoing medical treatment for some time, supported by her closest family and friends," the statement read. "She was a bright, beautiful and loving soul who cared deeply about others and who brought life to everything she touched. Her family asks for your understanding of their need for privacy at this time."
On July 13, John took to Instagram to share his heartbreak about his wife's death. He shared a photograph of her and wrote an emotional message paying tribute to his longtime partner, who he married in 1991.
View this 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. 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. Kelly’s love and life will always be remembered. 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. All my love, JT
"She fought a courageous fight with the love and support of so many," John wrote in his post. "My family and I will forever be grateful to her doctors and nurses at MD Anderson Cancer Center, all the medical centres that have helped, as well as her many friends and loved ones who have been by her side. Kelly's love and life will always be remembered.
"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. All my love, JT."
The stars were quick to post about their own heartbreak and sadness at Kelly's death.
"I am holding you all in love," Selma Blair wrote on John's Instagram tribute. "Your wife, your partner in life, was a darling soul. Always at the ready with comfort and grace. What a team you were. She will be so missed."
"Kelly was such a bright, loving soul, a talented actress, and a loving mom and wife," she tweeted. "My heart breaks for her family, who have already known such sadness and grief. Join me in sending them love and strength."
Daniel Dae Kim fondly remembered some of his time working with Kelly on 1999's For Love of the Game.
"We were shooting on cold days in Colorado, but she couldn't have been warmer or kinder to a young, nervous actor trying to make good," he tweeted. "I'll never forget that. Thank you, Kelly."
Ella, Kelly and John's 20-year-old daughter, also shared the same photo of her mother on her own Instagram as she posted a thoughtful tribute to her late parent.
"I have never met anyone as courageous, strong, beautiful and loving as you," she wrote. "Anyone who is lucky enough to have known you or to ever have been in your presence will agree that you have a glow and a light that never ceases to shine and that makes anyone around you feel instantly happy.
"Thank you for being there for me no matter what. Thank you for your love. Thank you for your help and thank you for making this world a better place. You have made life so beautiful and I know you will continue to do so always. I love you so much, mama."
Kelly was born in 1962 in Honolulu, Hawaii and went on to study acting at the University of Southern California. Her first film role was in Mischief, and from there, she moved into television. That was followed by spots in movies such as SpaceCamp, Twins, Jerry Maguire, Battlefield Earth alongside John, What a Girl Wants, The Cat in the Hat and more.
John and Kelly married in 1991, and she gave birth to their son, Jett, in 1992. Their daughter Ella followed in 2000 and their youngest son, Ben, came into the world in 2010. Jett sadly passed away in 2009 at age 16.