In 1953, Johnston was a lab technician in the potato development laboratory at the Ontario Agriculture College and he led a team that cross-bred two varieties to create the new type. In 1959, one of Johnston’s graduate students, a young man originally from Peru, told him of a small, rough, deep-yellow-fleshed potato ("Solanum goniocalyx", known as "", Spanish for "yellow potato") that was grown by the many indigenous communities in the Peruvian Andes. In Lima, this cultivar is considered a delicacy for its bright colour and distinct flavour. After trying these Peruvian potatoes, Johnston set out to breed a potato with the same colour and flavor characteristics, but larger in size and with a smoother shape, similar to the potatoes being grown in that part of Southwestern Ontario. In 1966, the development team made their first cross between a W5289-4 (2× cross between 'Yema de huevo' and 2× Katahdin) and a 'Norgleam' potato native to North Dakota. After the 66th cross that year, a true-breeding seed was produced, and the G6666 was created.