For many centuries, small breeds of spaniels have been popular in the United Kingdom. In the eleventh century, during the reign of King Canute, it was illegal to hunt with any dog that could not fit through a gauge that was eleven inches in diameter. Hence, the "birth" of the Toy Spaniel in the United Kingdom. Some centuries later, Toy Spaniels became popular as pets, especially as pets of the royal family. In fact, the Cavalier King Charles Spaniel was so named because a Blenheim coated spaniel was the children's pet in the household of Charles I. King Charles II went so far as to issue a decree that a Cavalier King Charles Spaniel could not be forbidden entrance to any public place, including the Houses of Parliament. Many Cavalier King Charles Spaniels can be seen in many paintings of the 16th, 17th and 18th centuries. These early cavaliers had longer, pointier snouts and thinner-boned limbs than today's.

Over time, the toy spaniels were replaced in popularity by short-snouted, dome-headed dogs of Asian descent, such as the Pug and Japanese Chin. The King Charles Spaniel was bred with these dogs, resulting in the similar-shaped head of today's English Toy Spaniel breed. The King Charles Spaniel remained popular at Blenheim Palace, home to the Dukes of Marlborough, where the brown and white version was the most popular - resulting in the name Blenheim for that color combination.
In the 1920s, an American named Roswell Eldrige offered twenty-five pounds as a prize for any King Charles Spaniel "of the old-fashioned type" with a longer nose, flat skull, and a lozenge (spot) in the middle of the crown of the head, sometimes called "the kiss of Buddha," "Blenheim Spot," "lozenge" or "Kissing Spot". So, the breed was developed by selective breeding of short-snouted Spaniels. The result was a dog that resembled the boyhood pet of the future Charles II of England ("Cavalier King Charles"), hence the name of the breed.

The Cavalier King Charles Spaniel (along with the Pug) is perhaps the largest toy breed: though clearly a lap dog, fully-grown adults tend to fill one rather amply. It is nonetheless quite small for a spaniel, when fully-grown a Cavalier King Charles Spaniels is roughly comparable in size to an adolescent of a more conventional spaniel breed. Breed standards call for a height between 29 and 33 cm (12–13 inches) with a proportionate weight between 4.5 and 8.5 kg (10 and 18 lb). Unlike most other spaniels, the Cavalier King Charles Spaniel has a full-length tail well-feathered with long hair, which is typically carried aloft when walking.