On July 1, 1959, the units of length in the United States and the Commonwealth of Nations were altered to become identical to the units which had, up to then, been adopted in Canada. The new units were all defined in terms of the International yard, defined as exactly 0.9144 m, slightly shorter than the previous definition in the United States, but slightly longer than the previous definition in the United Kingdom. (The differences were of the order of parts per million, and therefore to all intents and purposes were identical. But where extreme precision was wanted, the difference was accounted for.)
The International (statute) mile was in turn defined as 1760 International yards, which makes it exactly 1609.344 m. The previous United States customary mile and British Imperial mile were revised to correspond to the new definitions.