- Older half-brother of actress Drew Barrymore, half-brother of Blyth Dolores Barrymore (1 September 1962) and Jessica Barrymore (31 July 1966 - 29 July 2014).
- Son of John Drew Barrymore, grandson of legendary actor John Barrymore and Dolores Costello, & grand-nephew of legendary actors Lionel Barrymore & Ethel Barrymore and Helene Costello.
- His mother is the film and television actress Cara Williams who starred in the TV series Pete and Gladys (1960) and The Cara Williams Show (1964) and the film La Chaîne (1958) where she earned an Oscar nomination for Best Actress in a Supporting Role for her portrayal of 'Billy's mother'.
- John now works in the information technology field, particularly software development. (December 2006)
- Uncle of Olive and Frankie Barrymore Kopelman, Drew's daughters.
- Nephew of actress Diana Barrymore and DeeDee Barrymore.
- Father of John Sidney Blyth Barrymore IV with Jacqueline Barrymore and Sabrina Brooke & Blythe Lane with Rebecca Pogrow the second.
- Former brother-in-law of Will Kopelman, ex-husband of his half-sister Drew.
- Brother-in-law of Antonio Gioffredi (Blyth Dolores' Husband). Uncle of Gabriella and Nicole Gioffredi, their daughters.
- Cousin of Tony Barrymore.
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