On January 24, 1984, Apple Computer Inc. (now Apple Inc.) introduced the Macintosh personal computer, with the Macintosh 128K model, which came bundled with what was later renamed the Mac OS, but then known simply as the System Software. The Macintosh is generally credited with popularizing the graphical user interface. The Mac OS has been pre-installed on almost every Macintosh computer sold. The operating system is also sold separately on the Mac App Store for $29.99. It is also sold on a USB flash drive for $69.00. (Mac OS X Lion only.) The original Macintosh system software was partially based on the Lisa OS, previously released by Apple for the Lisa computer in 1983 and, as part of an agreement allowing Xerox to buy shares in Apple at a favorable rate, it also used concepts from the Xerox PARC Xerox Alto, which Steve Jobs and several other Macintosh team members had previewed.

The Macintosh project started in early 1979 with Jef Raskin, who envisioned an easy-to-use, low-cost computer for the average consumer. In September 1979, Raskin began looking for an engineer who could put together a prototype. Bill Atkinson, a member of the Apple Lisa team, introduced him to Burrell Smith, a service technician who had been hired earlier that year.

Mac OS is a series of graphical user interface-based operating systems developed by Apple Inc. (formerly Apple Computer, Inc.) for their Macintosh line of computer systems. The Macintosh user experience is credited with popularizing the graphical user interface. The original form of what Apple would later name the "Mac OS" was the integral and unnamed system software first introduced in 1984 with the original Macintosh, usually referred to simply as the System software.

From the beginning, Apple deliberately sought to minimize by design the user's conceptual awareness of the operating system as such. Tasks that on other products required a more explicit working knowledge of an operating system would on a Macintosh be accomplished by intuitive mouse gestures and manipulation of graphical control panels. The intention was that the product would thus be more user-friendly and so more easily mastered. This would differentiate it from devices using other operating environments, such as MS-DOS machines, which were more technically challenging to operate.