It achieved golden master status on July 9, 2012. OS X Mountain Lion was officially announced by Apple on their website on February 16, 2012, as a successor to OS X Lion. OS X Mountain Lion was announced at WWDC 2012 at Moscone West. The same practice was also applied to its predecessor, Mac OS X Lion. Apple later allowed free downloads of the OS, especially for customers of older and no longer officially supported Macintosh computers, starting on June 30, 2021. Mountain Lion was the last paid upgrade for an OS X major release, with OS X Mavericks and later being free. Mountain Lion sold three million units in the first four days, and has sold 28 million units as of June 10, 2013, making it Apple's most popular OS X release. OS X Mountain Lion received positive reviews, with critics praising Notification Center, Messages, and speed improvements over Mac OS X Lion, while criticizing iCloud for unreliability and Game Center for lack of games.

It was released as a downloadable update later. Facebook integration was also planned but unfinished at launch date. Integrated links allowing the user to rapidly transfer content to Twitter were present in the operating system from launch. Mountain Lion also added a version of iOS's Notification Center, which groups updates from different applications in one place. As on iOS, Notes and Reminders became full applications, separate from Mail and Calendar, while the iChat application was replaced with a version of iOS's Messages. The operating system gained the new malware-blocking system Gatekeeper and integration with Apple's online Game Center and iCloud services, while the Safari web browser was updated to version 6. Named to signify its status as a refinement of the previous OS X version, Lion, Apple's stated aims in developing Mountain Lion were to allow users to more easily manage and synchronise content between multiple Apple devices and to make the operating system more familiar.

OS X Mountain Lion was released on July 25, 2012, for purchase and download through Apple's Mac App Store, as part of a switch to releasing OS X versions online and every year, rather than every two years or so. OS X Mountain Lion (version 10.8) is the ninth major release of macOS, Apple Inc.'s desktop and server operating system for Macintosh computers.

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