Around 200 years ago, John Ellman in Sussex began improving the breed by selection within the breed - no outcrossing.  His work was continued by Jonas Webb who bred a somewhat larger animal which was used in the creation of the other Down breeds.  In the nineteenth and twentieth centuries they were huge numbers of sheep in and around the area of the South Downs near Lewes in Sussex, and it was from here they gained their name.  It has been estimated that there were some 110,000 sheep in Sussex in 1341.  In 1813, nearly five centuries later, the Reverend Arthur Young estimated that there were 200,000 ewes kept on the eastern South Downs.  It is thought that the Southdown sheep we know today originated from the sheep that grazed these downs centuries ago.

The peak of the Southdown's popularity was from about 1790 to the 1914-18 War.  During this time they were to be found on many of the large estates in flocks of 1000 or over, as well as being in the possession of yeoman farmers.  Although it had been exported to many parts of the world, it probably had it's greatest impact in New Zealand, where it was the sire used in the production of the "Canterbury Lamb", a position it held for many decades.

The introduction of the Southdown to the United States possibly occurred in the 1600s, when the sheep were brought to Jamestown, Virginia in 1609 and to Plymouth, Massachusetts in 1628.  By 1676 sheep were plentiful in New England, and were considered 'native' to the area.  However, these sheep were known to have descended from importations from 1628 and later.  These "native sheep" were called "Irish" or "English Smuts", and were probably descendants from the old English Southdowns.  By 1900, Southdowns were well established in the United States.  The size and length of the Southdown sheep is much larger today than 300 years ago due to successful attempts at improving the breed.