Lennon, who was married to the Beatles musician from 1962 to 1968, died Wednesday at her home in Mallorca, Spain, after battling cancer.
"The news of Cynthia’s passing is very sad," Paul McCartney told PEOPLE. "She was a lovely lady who I’ve known since our early days together in Liverpool. She was a good mother to Julian and will be missed by us all but I will always have great memories of our times together."
"Her son Julian Lennon was at her bedside throughout," a representative wrote in a statement.
Cynthia, a salesman's daughter who grew up in Hoylake, England, met the singer-songwriter while both were students at the Liverpool College of Art. The couple wed in 1962 after she became pregnant with their only child, Julian, who was born the following year.
"I got pregnant, we got married, the Beatles began – and we were all plunged into the whirlwind," she explained to PEOPLE in 1981.
Cynthia and John divorced in 1968, and a year later he wed Yoko Ono, to whom he remained married until he was fatally shot in December 1980.
Cynthia published two books that detailed her relationship with the music legend. In 1978, she published a book of memoirs titled A Twist of Lennon, followed by a more recent intimate biography, John, in 2005.
Her musician son, 51, announced his mother's death via a statement on social media, along with a video tribute to his late mother on his YouTube page.