It is principally a white-bodied, rough-coated breed, developed in the mid-to-late-19th century by Captain John Edwardes at Sealyham House, Pembrokeshire.

This decline has been blamed on an influx of foreign and designer breeds, and the Sealyham's reduced usefulness as a working dog.

A DNA test is now readily available to identify dogs who carry the gene that causes lens luxation and breeding programmes can be adjusted.

The breed was developed between 1850 and 1891 by Captain John Edwardes, at Sealyham House, near Wolfscastle in the Welsh county of Pembrokeshire.

Originally the breed was used for pest control, to hunt small game, and to eliminate vermin, particularly badgers.

[3] After Edwardes' death in 1891, other breeders began to work with Sealyhams,[2] including Fred Lewis, who promoted the breed.

The terrier was owned by actors Tallulah Bankhead, Humphrey Bogart, Bette Davis, Elizabeth Taylor,[1] and by writer Agatha Christie.

"[10] These two dogs were called Pippin and Johnny, and were looked after by the Queen Mother when Princess Margaret fell ill.[1] In the 1960s, children's author Maurice Sendak owned a Sealyham named Jennie, which he featured in his 1967 work Higglety Pigglety Pop!.

[4] In 2008, registrations of new puppies with the Kennel Club dropped to an all-time low of 43, placing it among the bottom three on the list of Vulnerable Native Breeds.

[1] End of year figures for 2011 showed that 49 puppies were registered with the Kennel Club in the UK, keeping them within the bottom three on the list of most endangered breeds.

[14] Harry Parsons, founder of the Working Sealyham Terrier Club, has stated that, "To sustain a breed...you need between 300 to 500 puppies a year".

The Kennel Club breed standard states the height of a Sealyham Terrier should not exceed 31 centimetres (12 in), measured at the withers or at the top of the shoulder blade.

"[2] A 2024 UK study found a life expectancy of 13.1 years for the breed compared to an average of 12.7 for purebreeds and 12 for crossbreeds.

[24] The Sealyham Terrier is one of the more commonly affected breeds for primary lens luxation which is caused by an autosomal recessive mutation of the ADAMTS17 gene.

[26] As of November 2011, the Kennel Club has not highlighted any specific concerns regarding the breed's health to conformation show judges.