The U.K.-born musician had been suffering from cancer and died surrounded by his family, his son Max said.
“He was incredibly brave and never lost his kindness or sense of humor even when things were tough,” he wrote on Facebook.
“My father was a brilliant and intuitive musician, a gentle soul and a wonderful father. He will live forever through his beautiful music and the love of his fans.”
McDonald wrote and played keyboards and saxophone on King Crimson’s iconic debut album, In the Court of the Crimson King.
The 1969 LP was described as “the big bang of progressive rock” by Rolling Stone.
American guitarist Steve Stevens called the album “the most important recording in progressive rock … at times dissonant chaos followed by aching beauty.”
McDonald went on to form Foreigner, playing guitar on such classic songs as Cold As Ice, Double Vision and Feels Like The First Time in the late 1970s.
But he left the band before finding new success in the 1980s with hits like Waiting for a Girl Like You and I Want to Know What Love Is.
Fellow Foreigner co-founder Al Greenwood paid tribute to him saying, “He was like a brother to me. A true musical genius, Ian’s musicianship was integral to launching King Crimson and Foreigner into legendary status.
“His contribution to Foreigner’s success was immense. Ian was a dear friend, a kind and wonderful man, and I will miss him terribly.”