It was shortly after WWII, in New Zealand, when Professor Ian Coop began to selectively crossbreed Border Leicester rams on Romney ewes. His purpose was to improve lambing percentages. The resulting crossbred sheep were interbred over several generations. Always, the selection and culling of progeny was based on recorded performance. By 1968, it was determined that the successive generations of the interbred sheep had produced a medium sized, highly prolific, dual purpose, adaptable sheep and it was officially recorded as the Coopworth breed.
Several large importations of Coopworths into the United States and Canada occurred in the 1970's and 1980's. These sheep were destined for both large, commercial operations and smaller, niche market producers and included white and natural colored animals. Today, due to costly quarantine regulations, artificial insemination is often used to obtain genetic diversity and continued improvement of the breed.