In television, the format dates back to at least a 1966 ABC broadcast of an adaptation of The Rise and Fall of the Third Reich. The series The Prisoner is another early example of the format, having been pitched by Patrick McGoohan for 9 episodes, and expanded to 17 due to studio concerns that such a short series would be difficult to sell.
The term became well-established in the early 1970s. Alex Haley's Roots in 1976 was the first blockbuster success of the format. Its success in the USA was due to its schedule: the twelve hours were split into eight episodes broadcast on consecutive nights, resulting in a finale with a 71 percent share of the audience and 130 million viewers.
In comic books, the format dates from the early 1980s, including The Phantom Zone published by DC Comics. Early miniseries were 3 or 4 issues in length, with longer series sometimes being termed "maxi-series". Sometimes a miniseries would be planned for a certain number of issues, but would run longer or shorter than originally planned. The series Sonic Disruptors was planned for 12 issues but was cancelled after #7.
The term has been falling out of use in comic books as of the 2000s due to the larger number of series which are published with a set length in mind.