During the early part of the 18th century, John Churchill, 1st Duke of Marlborough, kept red and white King Charles type spaniels for hunting. The duke recorded that they were able to keep up with a trotting horse. His estate was named Blenheim in honour of his victory at the Battle of Blenheim. Because of this influence, the red and white variety of the King Charles Spaniel and thus the Cavalier King Charles Spaniel became known as the Blenheim. Attempts were made to recreate the original Cavalier King Charles Spaniel as early as the turn of the 20th century, using the now extinct Toy Trawler Spaniels. These attempts were documented by Judith Blunt-Lytton, 16th Baroness Wentworth, in the book "Toy Dogs and Their Ancestors Including the History And Management of Toy Spaniels, Pekingese, Japanese and Pomeranians" published under the name of the "Hon. Mrs Neville Lytton" in 1911.
In 1926, the American Roswell Eldridge offered a dog show class prize of twenty-five pounds each as a prize for the best male and females of "Blenheim Spaniels of the old type, as shown in pictures of Charles II of England's time, long face, no stop, flat skull, not inclined to be domed, with spot in centre of skull." The breeders of the era were appalled, although several entered what they considered to be sub-par Cavalier King Charles Spaniels in the competition. Eldridge died before seeing his plan come to fruition, but several breeders believed in what he said and in 1928 the first Cavalier club was formed. The first standard was created, based on a dog named "Ann's Son" owned by Mostyn Walker, and the Kennel Club recognised the variety as "King Charles Spaniels, Cavalier type".
The Second World War caused a drastic setback to the emerging breed, with the vast majority of breeding stock destroyed because of wartime hardship and food shortages. For instance, in the Ttiweh Cavalier Kennel, the population of sixty dogs dropped to three during the 1940s. Following the war, just six dogs would survive as the new beginning from which all present-day Cavaliers descend. These dogs were Ann's Son, his litter brother Wizbang Timothy, Carlo of Ttiweh, Duce of Braemore, Kobba of Kuranda and Aristide of Ttiweh. The numbers increased gradually, and in 1945 the Kennel Club first recognised the breed in its own right as the Cavalier King Charles Spaniel.
The history of the breed in America is relatively recent. The first recorded Cavalier living in the United States was brought from the United Kingdom in 1956 by W. Lyon Brown, together with Elizabeth Spalding and other enthusiasts, she founded the Cavalier King Charles Club USA which continues to the present day. In 1994, the American Cavalier King Charles Spaniel Club was created by a group of breeders to apply for recognition by the American Kennel Club. The Cavalier would go on to be recognised by the American Kennel Club in 1995, and the ACKCSC became the parent club for Cavaliers.