In 1903, the Kennel Club tried to merge several spaniel breeds into one, but faced opposition that was later resolved by King Edward VII.
In 1904, the American Kennel Club combined four breeds into a single breed known as the English Toy Spaniel.
In her 1911 work, Judith Blunt-Lytton, the 16th Baroness Wentworth, suggested that Charles II took his sister's dogs after her death in 1670.
In July 1918, Grand Duchess Anastasia Nikolaevna of Russia owned a King Charles Spaniel during the shooting of the Romanov family. A King Charles Spaniel corpse was found at the believed site of the family's burning.
In 1926, American Roswell Eldridge held a competition that eventually led to the development of the Cavalier King Charles Spaniel.
In 1928, breeders formed the first breed club for the new Cavalier King Charles Spaniel.
In 1945, the Kennel Club recognised the Cavalier King Charles Spaniel breed.
In 1960, Princess Margaret attended Princess Anne's tenth birthday party with her King Charles Spaniel, Rolly.
In 1988, a Finnish breed club survey found patella luxation occurrences ranging from 5.3% to 50%.
In 1997, the American Kennel Club recognised the Cavalier King Charles Spaniel.
In 2003, due to low registration numbers, the King Charles Spaniel was identified as a Vulnerable Native Breed by the Kennel Club.
Since 2006, tail docking has been illegal in the UK and some other European countries.
In 2007, a Finnish breed club survey found patella luxation occurrences ranging from 5.3% to 50%.
In 2008, the BBC documentary "Pedigree Dogs Exposed" criticized the breeding practices of pedigree breeds, including the King Charles Spaniel, highlighting health issues like syringomyelia.
In 2010, the King Charles Spaniel was ranked 126th in popularity by the American Kennel Club, while the Cavalier was much more popular.