"Dear Kirstie, I am so grateful for our years together and for the two incredibly beautiful children and now grandchildren that we have," he wrote alongside an image of Alley grabbing onto his button-down shirt as he smiled at the camera.
"You will be missed. With love, Parker," he wrote in the post's closing.
The pair's relationship dates back to the 80s, following her marriage to Bob Alley from 1970 to 1977.
According to a PEOPLE profile, Alley and Stevenson met at a bar. While she didn't recognize him as the actor from The Hardy Boys, she recalled the evening vividly. "I saw him and said to my roommate, 'For him, I would die,' " she remembered.
Ron Galella, Ltd./Ron Galella Collection/Getty
The pair married on Dec. 22, 1983, making it her second marriage and his first. Years later, they welcomed their two children by adoption: William in 1992 and Lillie in 1995. In 1997, Alley and Stevenson divorced.
They later became grandparents in 2016 when True welcomed his first child, a son named Waylon Tripp Parker, and again in May 2021 after Lillie welcomed her first child, a son named Ripp Woodrow Graham.
William and Lillie remembered Alley as an "amazing mother" in their statement announcing her death.
"We are sad to inform you that our incredible, fierce and loving mother has passed away after a battle with cancer, only recently discovered," they said on Monday.
"She was surrounded by her closest family and fought with great strength, leaving us with a certainty of her never-ending joy of living and whatever adventures lie ahead," they continued. "As iconic as she was on screen, she was an even more amazing mother and grandmother."
The siblings also noted how their mother's "zest and passion for life, her children, grandchildren and her many animals, not to mention her eternal joy of creating, were unparalleled and leave us inspired to live life to the fullest just as she did."