In the late 1940s/early 1950s is when to-go packaging emerged. Less than 7% of the breweries that existed around 1949 offered take-home or carry-home as it was called then. Beer was primarily distributed in cases of 6 or 12 in wood crates up until this time.
Pabst Brewing is largely credited as the first brewer to package beer in six packs. Supposedly, the magic number 6 was a result of studies conducted to determine the ideal weight for the average housewife to carry home from the store. However, it has also been said that beer was first sold in packs of 6 because of size, not weight. A six pack of beer was the perfect size to fit in a standard paper grocery bag.
Whatever the original justification for selling beer in packs of 6, the phenomenon really took off in the mid-1950s and continues today.