The cattle egret was originally a bird of Africa and southern Asia, following large grazing mammals and catching the insects they flush. Its first recorded appearance in the New World was in the Guianas in 1877, most plausibly birds carried across the Atlantic on winds.
What followed was rapid. The species reached Florida in the 1940s, bred there in the 1950s, and spread through North America to southern Canada within a few decades, while also expanding through South America and, from Asia, into Australia and New Zealand. The engine of the spread was cattle ranching: clearing forest for pasture manufactured exactly the habitat the bird needed, and it simply moved into it.
The spread is often described as an invasion, which is imprecise. No human transport carried the bird across the Atlantic, and its American expansion was natural dispersal into habitat people had unintentionally made. Deliberate introductions did happen elsewhere — Hawaii received a release in 1959, and a 1933 release in Western Australia failed — but they account for little of the range. Evidence of harm to native herons is limited, and whether it competes meaningfully remains an open question.