John Twynam, a Hampshire farmer, crossed his then-Hampshire flock with Cotswold rams around 1829.

The resultant half-bred rams were compact and blocky animals, and from around 1835 were sold into six or more of what were to become the first recognised pedigree Hampshire Down flocks in the United Kingdom.

The Southdowns had long dark brown or black legs, matured early, produced the best of mutton and a fine quality of medium wool.

The horns of the original breed have disappeared; the face and legs have become dark, the frame has become more compact, the bones smaller, the back broader and straighter, the legs shorter, and the flesh and wool of better quality, while the superior hardiness and greater size, as well as the large head and Roman nose of the old breed, still remain.

The resultant mutton had a desirable proportion of fat and lean, and was juicy and fine flavoured; the lambs were large and were usually dropped early and fed for market.